app_meetme: Emit warning if conference not found.

Currently, if a user tries to access a non-dynamic
MeetMe conference and the conference is not found,
the call simply silent hangs up. There is no indication
to the user that anything went wrong at all.

This changes the relevant debug message to a warning
so that the user is notified of this invalidity.

ASTERISK-29954 #close

Change-Id: Iebcfae3755d00f2150d676ee211c57bc59530048

apps/app_meetme.c

@@ -4761,7 +4761,7 @@ static struct ast_conference *find_conf(struct ast_channel *chan, char *confno,
 				}
 			}
 			if (!var) {
-				ast_debug(1, "%s isn't a valid conference\n", confno);
+				ast_log(LOG_WARNING, "%s isn't a valid conference\n", confno);
 			}
 			ast_config_destroy(cfg);
 		}