Fix valgrind test failures for pre_exec_time

Summary:
Valgrind tests were failing due to pre_exec_time not being initialized
on all code paths for execute. This is intended since we only track
times on the DML requests. Hence, initialize the variable before reading
it.

Squash with D14883715

Test Plan: mtr

Reviewers: mung, yzha

Differential Revision: https://phabricator.intern.facebook.com/D15319856

Tasks: T43167423

Tags: mysql80

sql/sql_parse.cc

@@ -4650,9 +4650,12 @@ int mysql_execute_command(THD *thd, bool first_level, ulonglong *last_timer) {
     case SQLCOM_DROP_SRS:
 
       DBUG_ASSERT(lex->m_sql_cmd != nullptr);
+
+      /* The appropriate sql_cmd will set thd->pre_exec_time */
+      thd->pre_exec_time = 0;
       res = lex->m_sql_cmd->execute(thd);
 
-      if (last_timer) {
+      if (last_timer && thd->pre_exec_time != 0) {
         thd->status_var.pre_exec_time +=
             my_timer_difftime(*last_timer, thd->pre_exec_time);
         *last_timer = thd->pre_exec_time;