mpegts: Make sure we don't return uninitialized packets

This fixes crashes, where the demuxer could return 0 even if the returned AVPacket isn't initialized at all. This could happen if running into EOF or running out of probesize with non-seekable sources.

From FFmpeg commit df8aa45

mythtv/external/FFmpeg/libavformat/mpegts-mythtv.c

@@ -2930,6 +2930,7 @@ static int mpegts_read_packet(AVFormatContext *s,
     MpegTSContext *ts = s->priv_data;
     int ret, i;
 
+    pkt->size = -1;
     ts->pkt = pkt;
     ret = handle_packets(ts, 0);
     if (ret < 0) {
@@ -2948,6 +2949,8 @@ static int mpegts_read_packet(AVFormatContext *s,
         }
     }
 
+    if (!ret && pkt->size < 0)
+        ret = AVERROR(EINTR);
     return ret;
 }