res_pjsip_keepalive: Don't crash if PJSIP module is not loaded.

........

Merged revisions 431555 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@431556 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_pjsip_keepalive.c

@@ -240,6 +240,8 @@ static struct ast_sorcery_observer keepalive_global_observer = {
 
 static int load_module(void)
 {
+	CHECK_PJSIP_MODULE_LOADED();
+
 	ast_sorcery_observer_add(ast_sip_get_sorcery(), "global", &keepalive_global_observer);
 	ast_sorcery_reload_object(ast_sip_get_sorcery(), "global");
 	ast_module_ref(ast_module_info->self);