Don't unreference the SLA object if there is no SLA object in the devicestate callback. (issue #8354 reported by loloski)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@47748 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_meetme.c

@@ -2930,6 +2930,10 @@ static int slastate(const char *data)
 
 	/* Find conference */
 	sla = sla2 = ASTOBJ_CONTAINER_FIND(&slas, data);
+
+	if (!sla2)
+		return AST_DEVICE_INVALID;
+
 	ASTOBJ_UNREF(sla2, sla_destroy);
 
 	ast_log(LOG_DEBUG, "for '%s' conf = %p, sla = %p\n", data, conf, sla);