MDEV-22422: Assertion `! is_set()' failed in Diagnostics_area::set_eof_status

Analysis: The error is not returned when the statement can't be used.
And so we still go on to search for keywords.
Fix: Return the error state.

sql/sql_help.cc

@@ -741,6 +741,9 @@ static bool mysqld_help_internal(THD *thd, const char *mask)
 			      &name, &description, &example);
   delete select;
 
+  if (thd->is_error())
+    goto error;
+
   if (count_topics == 0)
   {
     int UNINIT_VAR(key_id);