Similar warnings during another run

==7142== Thread 40:
==7142== Syscall param pwrite64(buf) points to uninitialised byte(s)
==7142==    at 0x4E3DEC3: ??? (in /lib64/libpthread-2.12.so)
==7142==    by 0x7D6083: my_pwrite (my_pread.c:185)
==7142==    by 0x94B5D3: mi_nommap_pwrite (mysql_file.h:1201)
==7142==    by 0x94D5DD: _mi_write_part_record (mi_dynrec.c:766)
==7142==    by 0x94E27A: _mi_write_blob_record (mi_dynrec.c:370)
==7142==    by 0x9655A9: mi_write (mi_write.c:142)
==7142==    by 0x934E82: ha_myisam::write_row(unsigned char*) (ha_myisam.cc:831)
==7142==    by 0x69BBC1: handler::ha_write_row(unsigned char*) (handler.cc:5359)
==7142==    by 0x5ED848: select_union::send_data(List<Item>&) (sql_union.cc:67)
==7142==    by 0x5A4243: end_send(JOIN*, st_join_table*, bool) (sql_select.cc:12731)
==7142==    by 0x5AE31A: evaluate_join_record(JOIN*, st_join_table*, int) (sql_select.cc:11935)
==7142==    by 0x5AE40C: sub_select(JOIN*, st_join_table*, bool) (sql_select.cc:11788)
==7142==    by 0x5AE7DF: do_select(JOIN*, List<Item>*, TABLE*, Procedure*) (sql_select.cc:11553)
==7142==    by 0x5BDF47: JOIN::exec() (sql_select.cc:2401)
==7142==    by 0x5EF054: st_select_lex_unit::exec() (sql_union.cc:518)
==7142==    by 0x5EFD9C: mysql_union(THD*, LEX*, select_result*, st_select_lex_unit*, unsigned long) (sql_union.cc:38)
==7142==  Address 0xffadac3 is 35 bytes inside a block of size 115 alloc'd
==7142==    at 0x4C278FE: malloc (vg_replace_malloc.c:270)
==7142==    by 0x7D55F1: my_malloc (my_malloc.c:38)
==7142==    by 0x94E182: _mi_write_blob_record (mi_dynrec.c:284)
==7142==    by 0x9655A9: mi_write (mi_write.c:142)
==7142==    by 0x934E82: ha_myisam::write_row(unsigned char*) (ha_myisam.cc:831)
==7142==    by 0x69BBC1: handler::ha_write_row(unsigned char*) (handler.cc:5359)
==7142==    by 0x5ED848: select_union::send_data(List<Item>&) (sql_union.cc:67)
==7142==    by 0x5A4243: end_send(JOIN*, st_join_table*, bool) (sql_select.cc:12731)
==7142==    by 0x5AE31A: evaluate_join_record(JOIN*, st_join_table*, int) (sql_select.cc:11935)
==7142==    by 0x5AE40C: sub_select(JOIN*, st_join_table*, bool) (sql_select.cc:11788)
==7142==    by 0x5AE7DF: do_select(JOIN*, List<Item>*, TABLE*, Procedure*) (sql_select.cc:11553)
==7142==    by 0x5BDF47: JOIN::exec() (sql_select.cc:2401)
==7142==    by 0x5EF054: st_select_lex_unit::exec() (sql_union.cc:518)
==7142==    by 0x5EFD9C: mysql_union(THD*, LEX*, select_result*, st_select_lex_unit*, unsigned long) (sql_union.cc:38)
==7142==    by 0x5BFBF9: handle_select(THD*, LEX*, select_result*, unsigned long) (sql_select.cc:291)
==7142==    by 0x57CABC: execute_sqlcom_select(THD*, TABLE_LIST*) (sql_parse.cc:4741)
==7142==
==7142== Syscall param write(buf) points to uninitialised byte(s)
==7142==    at 0x4E3D4ED: ??? (in /lib64/libpthread-2.12.so)
==7142==    by 0x7D80E4: my_write (my_write.c:41)
==7142==    by 0x7C62C7: my_b_flush_io_cache (mysql_file.h:1134)
==7142==    by 0x7C637B: end_io_cache (mf_iocache.c:1851)
==7142==    by 0x94F64E: mi_extra (mi_extra.c:156)
==7142==    by 0x5ED5A4: select_union::flush() (sql_union.cc:87)
==7142==    by 0x5EF0C6: st_select_lex_unit::exec() (sql_union.cc:532)
==7142==    by 0x5EFD9C: mysql_union(THD*, LEX*, select_result*, st_select_lex_unit*, unsigned long) (sql_union.cc:38)
==7142==    by 0x5BFBF9: handle_select(THD*, LEX*, select_result*, unsigned long) (sql_select.cc:291)
==7142==    by 0x57CABC: execute_sqlcom_select(THD*, TABLE_LIST*) (sql_parse.cc:4741)
==7142==    by 0x57FC4D: mysql_execute_command(THD*) (sql_parse.cc:2284)
==7142==    by 0x583422: mysql_parse(THD*, char*, unsigned int, Parser_state*) (sql_parse.cc:5811)
==7142==    by 0x5848E9: dispatch_command(enum_server_command, THD*, char*, unsigned int) (sql_parse.cc:1061)
==7142==    by 0x584C75: do_command(THD*) (sql_parse.cc:788)
==7142==    by 0x6228B0: do_handle_one_connection(THD*) (sql_connect.cc:1484)
==7142==    by 0x6229A3: handle_one_connection (sql_connect.cc:1391)
==7142==  Address 0x1c11fb3f is 15 bytes inside a block of size 131,072 alloc'd
==7142==    at 0x4C278FE: malloc (vg_replace_malloc.c:270)
==7142==    by 0x7D55F1: my_malloc (my_malloc.c:38)
==7142==    by 0x7C45A2: init_io_cache (mf_iocache.c:232)
==7142==    by 0x94F1F8: mi_extra (mi_extra.c:137)
==7142==    by 0x5ED786: select_union::create_result_table(THD*, List<Item>*, bool, unsigned long long, char const*) (sql_union.cc:129)
==7142==    by 0x5EE7C8: st_select_lex_unit::prepare(THD*, select_result*, unsigned long) (sql_union.cc:371)
==7142==    by 0x5EFD8E: mysql_union(THD*, LEX*, select_result*, st_select_lex_unit*, unsigned long) (sql_union.cc:36)
==7142==    by 0x5BFBF9: handle_select(THD*, LEX*, select_result*, unsigned long) (sql_select.cc:291)
==7142==    by 0x57CABC: execute_sqlcom_select(THD*, TABLE_LIST*) (sql_parse.cc:4741)
==7142==    by 0x57FC4D: mysql_execute_command(THD*) (sql_parse.cc:2284)
==7142==    by 0x583422: mysql_parse(THD*, char*, unsigned int, Parser_state*) (sql_parse.cc:5811)
==7142==    by 0x5848E9: dispatch_command(enum_server_command, THD*, char*, unsigned int) (sql_parse.cc:1061)
==7142==    by 0x584C75: do_command(THD*) (sql_parse.cc:788)
==7142==    by 0x6228B0: do_handle_one_connection(THD*) (sql_connect.cc:1484)
==7142==    by 0x6229A3: handle_one_connection (sql_connect.cc:1391)
==7142==    by 0x4E36850: start_thread (in /lib64/libpthread-2.12.so)
==7142==

Also see: 
http://jorgenloland.blogspot.com.au/2012/04/copying-unused-bytes-is-bad-duh.html
Search for: mi_write_blob_record and see note re 5.6