Bug#37316: Anonymous error messages noticed sometimes, while running tests in MTR

The problem was that mysqltest could attempt to execute a
SHOW WARNINGS statement through a connection that was not
properly reaped, thus violating its own rules.

The solution is to skip SHOW WARNINGS if a connection has
not been properly repeaed.

client/mysqltest.cc:
  Skip SHOW WARNINGS if connection hasn't been reaped.

client/mysqltest.cc

@@ -1256,7 +1256,7 @@ void die(const char *fmt, ...)
     Help debugging by displaying any warnings that might have
     been produced prior to the error
   */
-  if (cur_con)
+  if (cur_con && !cur_con->pending)
     show_warnings_before_error(&cur_con->mysql);
 
   cleanup_and_exit(1);