Ensure that thd->user_var_events_alloc is always relevant

If binary logging is not enabled thd->user_var_events_alloc
would never be initialized.

sql/sp_head.cc

@@ -1135,6 +1135,7 @@ sp_head::execute(THD *thd, bool merge_da_on_success)
   uint old_server_status;
   const uint status_backup_mask= SERVER_STATUS_CURSOR_EXISTS |
                                  SERVER_STATUS_LAST_ROW_SENT;
+  MEM_ROOT *user_var_events_alloc_saved= 0;
   Reprepare_observer *save_reprepare_observer= thd->m_reprepare_observer;
   Object_creation_ctx *UNINIT_VAR(saved_creation_ctx);
   Diagnostics_area *da= thd->get_stmt_da();
@@ -1326,9 +1327,11 @@ sp_head::execute(THD *thd, bool merge_da_on_success)
       Will write this SP statement into binlog separately.
       TODO: consider changing the condition to "not inside event union".
     */
-    MEM_ROOT *user_var_events_alloc_saved= thd->user_var_events_alloc;
     if (thd->locked_tables_mode <= LTM_LOCK_TABLES)
+    {
+      user_var_events_alloc_saved= thd->user_var_events_alloc;
       thd->user_var_events_alloc= thd->mem_root;
+    }
 
     sql_digest_state *parent_digest= thd->m_digest;
     thd->m_digest= NULL;

sql/sql_class.cc

@@ -772,6 +772,12 @@ THD::THD(my_thread_id id, bool is_wsrep_applier)
   init_sql_alloc(&main_mem_root, ALLOC_ROOT_MIN_BLOCK_SIZE, 0,
                  MYF(MY_THREAD_SPECIFIC));
 
+  /*
+    Allocation of user variables for binary logging is always done with main
+    mem root
+  */
+  user_var_events_alloc= mem_root;
+
   stmt_arena= this;
   thread_stack= 0;
   scheduler= thread_scheduler;                 // Will be fixed later

sql/sql_parse.cc

@@ -7483,10 +7483,9 @@ void THD::reset_for_next_command(bool do_clear_error)
   thd->thread_specific_used= FALSE;
 
   if (opt_bin_log)
-  {
     reset_dynamic(&thd->user_var_events);
-    thd->user_var_events_alloc= thd->mem_root;
-  }
+  DBUG_ASSERT(thd->user_var_events_alloc == &thd->main_mem_root);
+
   thd->get_stmt_da()->reset_for_next_command();
   thd->rand_used= 0;
   thd->m_sent_row_count= thd->m_examined_row_count= 0;