sig_pri_new_ast_channel() should return NULL when new_ast_channel() fails.

(closes issue #18874)
Reported by: cmaj
Patches:
      patch-sig_pri-crash-possible-null-channel-pointer.diff.txt uploaded by cmaj (license 830)

JIRA SWP-3172


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@308622 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/sig_pri.c

@@ -865,10 +865,14 @@ static struct ast_channel *sig_pri_new_ast_channel(struct sig_pri_chan *p, int s
 {
 	struct ast_channel *c;
 
-	if (p->calls->new_ast_channel)
+	if (p->calls->new_ast_channel) {
 		c = p->calls->new_ast_channel(p->chan_pvt, state, ulaw, exten, requestor);
-	else
+	} else {
+		return NULL;
+	}
+	if (!c) {
 		return NULL;
+	}
 
 	if (!p->owner)
 		p->owner = c;