dropping table with corrupt table proto message results in segv

[Stewart Smith]

found in testing for create_internal_temporary_table.

corrupt table proto message will result in SIGSEGV:

#0 0x0000000000527507 in drizzled::plugin::StorageEngine::print_error (
    this=0x0, error=-1, errflag=0, table=0x0)
    at drizzled/plugin/storage_engine.cc:1241
#1 0x0000000000525b07 in drizzled::plugin::StorageEngine::dropTable (
    session=..., identifier=..., generate_warning=true)
    at drizzled/plugin/storage_engine.cc:751
#2 0x000000000058bda7 in mysql_rm_table_part2 (session=0xb6ee70,
    tables=0x1379a78, if_exists=false, drop_temporary=false)
    at drizzled/sql_table.cc:495
#3 0x00000000005b6692 in mysql_rm_table (session=0xb6ee70, tables=0x1379a78,
    if_exists=false, drop_temporary=false)
    at drizzled/statement/drop_table.cc:89
#4 0x00000000005b67a2 in drizzled::statement::DropTable::execute (
    this=0x1371410) at drizzled/statement/drop_table.cc:118
#5 0x0000000000574501 in mysql_execute_command (session=0xb6ee70)
    at drizzled/sql_parse.cc:501
#6 0x000000000057529b in mysql_parse (session=0xb6ee70,
    inBuf=0x13799f8 "DROP TABLE test.t1", length=18,
    found_semicolon=0x7fffe4f7ffc8) at drizzled/sql_parse.cc:790
#7 0x0000000000573cf2 in dispatch_command (command=COM_QUERY,
    session=0xb6ee70, packet=0x13719c1 "DROP TABLE test.t1", packet_length=18)
    at drizzled/sql_parse.cc:224
#8 0x00000000005376cf in Session::executeStatement (this=0xb6ee70)

as engine is NULL and we try and use it.