Server crashes in check_alter_user on setting a default role via PS

create role r1;

prepare stmt from "set default role r1";

execute stmt;

#3  <signal handler called>

#4  __strcmp_ssse3 () at ../sysdeps/x86_64/multiarch/../strcmp.S:214

#5  0x00007f02ecaf6da6 in check_alter_user (thd=0x7f02e77f8070, host=0x8f8f8f8f8f8f8f8f <Address 0x8f8f8f8f8f8f8f8f out of bounds>, user=0x8f8f8f8f8f8f8f8f <Address 0x8f8f8f8f8f8f8f8f out of bounds>) at 10.1/sql/sql_acl.cc:2584

#6  0x00007f02ecaf7700 in acl_check_set_default_role (thd=0x7f02e77f8070, host=0x8f8f8f8f8f8f8f8f <Address 0x8f8f8f8f8f8f8f8f out of bounds>, user=0x8f8f8f8f8f8f8f8f <Address 0x8f8f8f8f8f8f8f8f out of bounds>) at 10.1/sql/sql_acl.cc:2750

#7  0x00007f02ecadade2 in set_var_default_role::check (this=0x7f02e0f2d5d8, thd=0x7f02e77f8070) at 10.1/sql/set_var.cc:902

#8  0x00007f02ecada671 in sql_set_variables (thd=0x7f02e77f8070, var_list=0x7f02e0f2ce40) at 10.1/sql/set_var.cc:687

#9  0x00007f02ecb864c6 in mysql_execute_command (thd=0x7f02e77f8070) at 10.1/sql/sql_parse.cc:4103

#10 0x00007f02ecba8d70 in Prepared_statement::execute (this=0x7f02e0e49c70, expanded_query=0x7f02ec651550, open_cursor=false) at 10.1/sql/sql_prepare.cc:4038

#11 0x00007f02ecba7bb9 in Prepared_statement::execute_loop (this=0x7f02e0e49c70, expanded_query=0x7f02ec651550, open_cursor=false, packet=0x0, packet_end=0x0) at 10.1/sql/sql_prepare.cc:3665

#12 0x00007f02ecba5c55 in mysql_sql_stmt_execute (thd=0x7f02e77f8070) at 10.1/sql/sql_prepare.cc:2801

#13 0x00007f02ecb826b4 in mysql_execute_command (thd=0x7f02e77f8070) at 10.1/sql/sql_parse.cc:2813

#14 0x00007f02ecb8f241 in mysql_parse (thd=0x7f02e77f8070, rawbuf=0x7f02e0cff088 "execute stmt", length=12, parser_state=0x7f02ec6521c0) at 10.1/sql/sql_parse.cc:6953

#15 0x00007f02ecb7f301 in dispatch_command (command=COM_QUERY, thd=0x7f02e77f8070, packet=0x7f02e5ffa071 "execute stmt", packet_length=12) at 10.1/sql/sql_parse.cc:1466

#16 0x00007f02ecb7e11f in do_command (thd=0x7f02e77f8070) at 10.1/sql/sql_parse.cc:1095

#17 0x00007f02eccabae7 in do_handle_one_connection (thd_arg=0x7f02e77f8070) at 10.1/sql/sql_connect.cc:1351

#18 0x00007f02eccab82c in handle_one_connection (arg=0x7f02e77f8070) at 10.1/sql/sql_connect.cc:1262

#19 0x00007f02ed23cf6e in pfs_spawn_thread (arg=0x7f02e98249f0) at 10.1/storage/perfschema/pfs.cc:1860

#20 0x00007f02ec288b50 in start_thread (arg=<optimized out>) at pthread_create.c:304

#21 0x00007f02ea31f20d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112

commit 9646f946e912db02fb75493b48db188476860c8e

Author: Jan Lindström <jan.lindstrom@skysql.com>

Date:   Mon Oct 27 16:34:53 2014 +0200

    MDEV-6759: innodb valgrind failures

    Fix failure seen on dict_foreign_remove_partial.