add a check to prevent a seg fault from an unknown cause ... (bug #3496)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/v1-0@5122 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_skinny.c

@@ -1576,7 +1576,8 @@ static int skinny_answer(struct ast_channel *ast)
 static struct ast_frame *skinny_rtp_read(struct skinny_subchannel *sub)
 {
 	/* Retrieve audio/etc from channel.  Assumes sub->lock is already held. */
-	struct ast_frame *f;
+	struct ast_frame *f = NULL;
+	if (sub->rtp) {	
 		f = ast_rtp_read(sub->rtp);
 		if (sub->owner) {
 			/* We already hold the channel lock */
@@ -1589,6 +1590,8 @@ static struct ast_frame *skinny_rtp_read(struct skinny_subchannel *sub)
 				}
 			}
 		}
+	} else
+		ast_log(LOG_WARNING, "sub->rtp is NULL - calling read on a channel that has been hungup?\n");
 	return f;
 }