Fix a crash in res_pjsip_t38 caused by the wrong assumption that a session will always have a channel.

When starting up or shutting down this assumption is false.
........

Merged revisions 400284 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400285 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_pjsip_t38.c

@@ -446,7 +446,8 @@ static void t38_attach_framehook(struct ast_sip_session *session)
 		.event_cb = t38_framehook,
 	};
 
-	if ((ast_channel_state(session->channel) == AST_STATE_UP) || !session->endpoint->media.t38.enabled) {
+	if (!session->channel || (ast_channel_state(session->channel) == AST_STATE_UP) ||
+	    !session->endpoint->media.t38.enabled) {
 		return;
 	}