Crashed PS 5.6.17 on Ubuntu 13.10 easily:

openxs@ao756:~/dbs/p5.6$ mysql -uroot -proot test
Warning: Using a password on the command line interface can be insecure.
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 47
Server version: 5.6.17-65.0-587.saucy (Ubuntu)

Copyright (c) 2009-2014 Percona LLC and/or its affiliates
Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> drop table if exists sometable;
Query OK, 0 rows affected (0,21 sec)

mysql> CREATE TABLE sometable
    -> ( key1 INT UNSIGNED NOT NULL
    -> , key2 INT UNSIGNED DEFAULT NULL
    -> , key3 CHAR(20) NOT NULL
    -> , data CHAR(80) NOT NULL
    -> , UNIQUE KEY key3_key1_key2 (key3, key1, key2)
    -> , KEY (key1)
    -> , KEY (key2)
    -> , KEY (key3)
    -> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0,89 sec)

mysql> ALTER TABLE sometable MODIFY key2 INT UNSIGNED NOT NULL DEFAULT 0;
Query OK, 0 rows affected, 1 warning (1,94 sec)
Records: 0  Duplicates: 0  Warnings: 1

mysql> show warnings\G
*************************** 1. row ***************************
  Level: Warning
   Code: 1082
Message: InnoDB: Table ./test/sometable has no primary key in InnoDB data dictionary, but has one in MySQL!
1 row in set (0,00 sec)

mysql> SELECT data FROM sometable WHERE key1 = 1 AND key3 = 3 AND key2 = 2;
ERROR 2013 (HY000): Lost connection to MySQL server during query
mysql> exit
Bye
openxs@ao756:~/dbs/p5.6$ sudo tail -80 /var/lib/mysql/ao756.err
key_buffer_size=8388608
read_buffer_size=131072
max_used_connections=2
max_threads=153
thread_count=1
connection_count=1
It is possible that mysqld could use up to
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 69163 K  bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x7f9e8e0dd930
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
stack_bottom = 7f9e5c781e10 thread_stack 0x40000
/usr/sbin/mysqld(my_print_stacktrace+0x2c)[0x7f9e8bdec10c]
/usr/sbin/mysqld(handle_fatal_signal+0x3cb)[0x7f9e8bb3942b]
/lib/x86_64-linux-gnu/libpthread.so.0(+0xfbb0)[0x7f9e8a001bb0]
/usr/sbin/mysqld(+0x8c76a8)[0x7f9e8bf9f6a8]
/usr/sbin/mysqld(+0x779f85)[0x7f9e8be51f85]
/usr/sbin/mysqld(_ZN7handler27multi_range_read_info_constEjP15st_range_seq_ifPvjPjS3_P13Cost_estimate+0xcf)[0x7f9e8ba6472f]
/usr/sbin/mysqld(_ZN10DsMrr_impl16dsmrr_info_constEjP15st_range_seq_ifPvjPjS3_P13Cost_estimate+0x48)[0x7f9e8ba6db58]
/usr/sbin/mysqld(+0x618318)[0x7f9e8bcf0318]
/usr/sbin/mysqld(_ZN10SQL_SELECT17test_quick_selectEP3THD6BitmapILj64EEyybN8st_order10enum_orderE+0xae0)[0x7f9e8bcff0f0]
/usr/sbin/mysqld(+0x66aef0)[0x7f9e8bd42ef0]
/usr/sbin/mysqld(_ZN4JOIN8optimizeEv+0x7d9)[0x7f9e8bd456b9]
/usr/sbin/mysqld(_Z12mysql_selectP3THDP10TABLE_LISTjR4ListI4ItemEPS4_P10SQL_I_ListI8st_orderESB_S7_yP13select_resultP18st_select_lex_unitP13st_select_lex+0x1f6)[0x7f9e8bbee4c6]
/usr/sbin/mysqld(_Z13handle_selectP3THDP13select_resultm+0x165)[0x7f9e8bbeedf5]
/usr/sbin/mysqld(+0x3518ba)[0x7f9e8ba298ba]
/usr/sbin/mysqld(_Z21mysql_execute_commandP3THD+0x2086)[0x7f9e8bbca3d6]
/usr/sbin/mysqld(_Z11mysql_parseP3THDPcjP12Parser_state+0x618)[0x7f9e8bbcf568]
/usr/sbin/mysqld(_Z16dispatch_command19enum_server_commandP3THDPcj+0x237b)[0x7f9e8bbd1f6b]
/usr/sbin/mysqld(_Z24do_handle_one_connectionP3THD+0x22d)[0x7f9e8bb9699d]
/usr/sbin/mysqld(handle_one_connection+0x40)[0x7f9e8bb96a20]
/usr/sbin/mysqld(pfs_spawn_thread+0x140)[0x7f9e8be2f9e0]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x7f6e)[0x7f9e89ff9f6e]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7f9e8971c9cd]

Trying to get some variables.
Some pointers may be invalid and cause the dump to abort.
Query (7f9e44005210): is an invalid pointer
Connection ID (thread ID): 47
Status: NOT_KILLED

...
openxs@ao756:~/dbs/p5.6$

No crash with PS 5.5.37 there or with older -debug build of PS 5.6.15