Better handle SIP devices that say they have SDP content... but really don't. (issue #9398 reported by mthomasslo)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@67068 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -4703,7 +4703,7 @@ static int find_sdp(struct sip_request *req)
 	if (!strcasecmp(content_type, "application/sdp")) {
 		req->sdp_start = 0;
 		req->sdp_end = req->lines;
-		return 1;
+		return req->lines ? 1 : 0;
 	}
 
 	/* if it's not multipart/mixed, there cannot be an SDP */