Modify ALTER USER processing when executing thread is Galera applier thread to correct assertion

#0  __pthread_kill (threadid=<optimized out>, signo=11) at ../sysdeps/unix/sysv/linux/pthread_kill.c:62
#1  0x000000000475d0a3 in my_write_core (sig=11) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/mysys/stacktrace.cc:278
#2  0x000000000359acc4 in handle_fatal_signal (sig=11) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/signal_handler.cc:264
#3  <signal handler called>
#4  __strcmp_sse2_unaligned () at ../sysdeps/x86_64/multiarch/strcmp-sse2-unaligned.S:31
#5  0x00000000036ce720 in validate_password_require_current (thd=0x7feebac0f000, Str=0x7fee78caea00, acl_user=0x7fee7f65e580, auth=0x6f47ba0 <native_password_handler>, is_privileged_user=true, user_exists=true) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/auth/sql_user.cc:740
#6  0x00000000036cf9f2 in set_and_validate_user_attributes (thd=0x7feebac0f000, Str=0x7fee78caea00, what_to_set=..., is_privileged_user=true, is_role=false, history_table=0x7feee2a4a318, history_check_done=0x7feee2a46e3b, cmd=0x5a8a07a "ALTER USER") at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/auth/sql_user.cc:1168
#7  0x00000000036d3fbd in mysql_alter_user (thd=0x7feebac0f000, list=..., if_exists=false) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/auth/sql_user.cc:2434
#8  0x00000000033b9bb4 in mysql_execute_command (thd=0x7feebac0f000, first_level=true) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/sql_parse.cc:5342
#9  0x00000000033bc297 in mysql_parse (thd=0x7feebac0f000, parser_state=0x7feee2a4cca0, update_userstat=true, force_primary_storage_engine=true) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/sql_parse.cc:6076
#10 0x00000000042f76d2 in Query_log_event::do_apply_event (this=0x7feebad2a020, rli=0x7feebaca2000, query_arg=0x7feebaed3f19 "ALTER USER u1 IDENTIFIED BY '' REPLACE '' PASSWORD REQUIRE CURRENT", q_len_arg=66) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/log_event.cc:4669
#11 0x00000000042f5f09 in Query_log_event::do_apply_event (this=0x7feebad2a020, rli=0x7feebaca2000) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/log_event.cc:4333
#12 0x00000000042f1c3a in Log_event::apply_event (this=0x7feebad2a020, rli=0x7feebaca2000) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/log_event.cc:3172
#13 0x00000000035d6dc2 in wsrep_apply_events (thd=0x7feebac0f000, events_buf=0x7feec29358a8, buf_len=0) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/wsrep_applier.cc:176
#14 0x00000000035d79ac in wsrep_apply_cb (ctx=0x7feebac0f000, buf=0x7feec29358a8, buf_len=135, flags=5, meta=0x7feee2a4ff80) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/wsrep_applier.cc:302
#15 0x00007feecac8d048 in galera::TrxHandle::apply (this=0x7feea2320600, recv_ctx=0x7feebac0f000, apply_cb=0x35d7501 <wsrep_apply_cb(void*, void const*, unsigned long, unsigned int, wsrep_trx_meta const*)>, meta=...) at galera/src/trx_handle.cpp:317
#16 0x00007feecaccf348 in apply_trx_ws (recv_ctx=0x7feebac0f000, apply_cb=0x35d7501 <wsrep_apply_cb(void*, void const*, unsigned long, unsigned int, wsrep_trx_meta const*)>, commit_cb=0x35d8939 <wsrep_commit_cb(void*, void const*, unsigned int, wsrep_trx_meta const*, bool*, bool)>, trx=..., meta=...) at galera/src/replicator_smm.cpp:34
#17 0x00007feecacd2659 in galera::ReplicatorSMM::apply_trx (this=0x7feed2bf5000, recv_ctx=0x7feebac0f000, trx=0x7feea2320600) at galera/src/replicator_smm.cpp:494
#18 0x00007feecacd786f in galera::ReplicatorSMM::process_trx (this=0x7feed2bf5000, recv_ctx=0x7feebac0f000, trx=0x7feea2320600) at galera/src/replicator_smm.cpp:1419
#19 0x00007feecacb15f1 in galera::GcsActionSource::dispatch (this=0x7feed2bf56a8, recv_ctx=0x7feebac0f000, act=..., [email protected]: false) at galera/src/gcs_action_source.cpp:116
#20 0x00007feecacb1dc1 in galera::GcsActionSource::process (this=0x7feed2bf56a8, recv_ctx=0x7feebac0f000, [email protected]: false) at galera/src/gcs_action_source.cpp:181
#21 0x00007feecacd1c72 in galera::ReplicatorSMM::async_recv (this=0x7feed2bf5000, recv_ctx=0x7feebac0f000) at galera/src/replicator_smm.cpp:408
#22 0x00007feecacf4a26 in galera_recv (gh=0x7feedfc58600, recv_ctx=0x7feebac0f000) at galera/src/wsrep_provider.cpp:244
#23 0x00000000035dbdaa in wsrep_replication_process (thd=0x7feebac0f000) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/wsrep_thd.cc:447
#24 0x00000000031bc351 in start_wsrep_THD (arg=0x35dbcb1 <wsrep_replication_process(THD*)>) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/sql/mysqld.cc:8795
#25 0x0000000004853dc6 in pfs_spawn_thread (arg=0x7feed2be4920) at /mnt/workspace/qa.pxc80.build/BUILD_TYPE/debug/Host/min-xenial-x64/storage/perfschema/pfs.cc:2836
#26 0x00007feee244a6ba in start_thread (arg=0x7feee2a53700) at pthread_create.c:333
#27 0x00007feee07fe41d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109