A cleanup for MDEV-18333 Slow_queries count doesn't increase when slow_query_log is turned off

thd->lex->m_sql_cmd was not cleared between queries.
log_slow_query() could crash (when running mtr --ps) because of this.

sql/sql_parse.cc

@@ -1989,6 +1989,11 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
   dec_thread_running();
   thd->packet.shrink(thd->variables.net_buffer_length);	// Reclaim some memory
   thd->reset_kill_query();  /* Ensure that killed_errmsg is released */
+  /*
+    LEX::m_sql_cmd can point to Sql_cmd allocated on thd->mem_root.
+    Unlink it now, before freeing the root.
+  */
+  thd->lex->m_sql_cmd= NULL;
   free_root(thd->mem_root,MYF(MY_KEEP_PREALLOC));
 
 #if defined(ENABLED_PROFILING)