crash in drizzled::optimizer::ExplainPlan::printPlan

Found after fixing bug #628398 using the same randgen pattern:

./gentest.pl --dsn=dbi:drizzle:host=localhost:port=9306:user=root:password=:database=test --threads=1 --engine=Innodb --gendata=conf/drizzle/drizzle.zz --grammar=conf/drizzle/proclist_subquery_drizzle.yy --debug --queries=100000 --threads=2

drizzled::optimizer::ExplainPlan::printPlan (this=0x7fffd67fa560)
    at drizzled/optimizer/explain_plan.cc:229
229 strlen(real_table->alias),
(gdb) bt
#0 drizzled::optimizer::ExplainPlan::printPlan (this=0x7fffd67fa560)
    at drizzled/optimizer/explain_plan.cc:229
#1 0x000000000062c1df in drizzled::Join::exec (this=0x7fffc45c5e80)
    at drizzled/join.cc:1303
#2 0x00000000006b7691 in drizzled::select_query (session=0x1969690,
    rref_pointer_array=0x7fffc437e1d8, tables=0x7fffc43016d0,
    wild_num=<value optimized out>, fields=<value optimized out>,
    conds=<value optimized out>, og_num=2, order=0x0, group=0x7fffc4475880,
    having=0x0, select_options=2147500037, result=0x7fffc4476278, unit=
    0x7fffc437e290, select_lex=0x7fffc437e020) at drizzled/sql_select.cc:432
#3 0x000000000064dbbf in drizzled::optimizer::ExplainPlan::explainUnion (
    this=<value optimized out>, session=0x1969690, unit=0x7fffc437e290,
    result=0x7fffc4476278) at drizzled/optimizer/explain_plan.cc:579
#4 0x000000000064e0da in drizzled::optimizer::ExplainPlan::printPlan (
    this=0x7fffd67fb600) at drizzled/optimizer/explain_plan.cc:472
#5 0x000000000062c1df in drizzled::Join::exec (this=0x7fffc4532c08)
    at drizzled/join.cc:1303
#6 0x00000000006b7691 in drizzled::select_query (session=0x1969690,
    rref_pointer_array=0x196a468, tables=0x7fffc40054c8,
    wild_num=<value optimized out>, fields=<value optimized out>,
    conds=<value optimized out>, og_num=2, order=0x7fffc4476018, group=0x0,
    having=0x0, select_options=2147500037, result=0x7fffc4476278,
    unit=0x196a0b0, select_lex=0x196a2b0) at drizzled/sql_select.cc:432
#7 0x000000000064dbbf in drizzled::optimizer::ExplainPlan::explainUnion (
    this=<value optimized out>, session=0x1969690, unit=0x196a0b0,
    result=0x7fffc4476278) at drizzled/optimizer/explain_plan.cc:579
#8 0x00000000006b0b32 in drizzled::execute_sqlcom_select (session=0x1969690,
    all_tables=<value optimized out>) at drizzled/sql_parse.cc:520
#9 0x00000000006b3ab5 in execute_command (session=0x1969690,
    inBuf=<value optimized out>, length=1545) at drizzled/sql_parse.cc:479
#10 drizzled::parse (session=0x1969690, inBuf=<value optimized out>,
    length=1545) at drizzled/sql_parse.cc:750
#11 0x00000000006b418c in drizzled::dispatch_command (
    command=<value optimized out>, session=0x1969690, packet=0x196ca71 "",
    packet_length=1547) at drizzled/sql_parse.cc:227
#12 0x00000000006898d6 in drizzled::Session::executeStatement (this=0x1969690)
    at drizzled/session.cc:723
#13 0x000000000068cf57 in drizzled::Session::run (this=0x1969690)
    at drizzled/session.cc:564
#14 0x00007ffff56728a6 in multi_thread::MultiThreadScheduler::runSession (
    this=0x19462a0, id=4) at plugin/multi_thread/multi_thread.cc:66
#15 0x00007ffff789ba55 in thread_proxy ()
   from /usr/lib64/libboost_thread-mt.so.1.44.0
#16 0x0000003755406d5b in start_thread () from /lib64/libpthread.so.0
#17 0x00000037550e4a7d in clone () from /lib64/libc.so.6

Very likely duplicate of http://bugs.mysql.com/bug.php?id=54488