mpegtsenc: Only test the first frame for missing h264_mp4toannexb filter.

Many video players accept broken frames in a transport stream, so there
is no reason to abort remuxing when encountering one, just print a
warning instead.

Fixes ticket #1758.

libavformat/mpegtsenc.c

@@ -1095,10 +1095,13 @@ static int mpegts_write_packet_internal(AVFormatContext *s, AVPacket *pkt)
         uint32_t state = -1;
 
         if (pkt->size < 5 || AV_RB32(pkt->data) != 0x0000001) {
+            if (!st->nb_frames) {
             av_log(s, AV_LOG_ERROR, "H.264 bitstream malformed, "
                    "no startcode found, use the h264_mp4toannexb bitstream filter (-bsf h264_mp4toannexb)\n");
             return AVERROR(EINVAL);
             }
+            av_log(s, AV_LOG_WARNING, "H.264 bitstream error, startcode missing\n");
+        }
 
         do {
             p = avpriv_find_start_code(p, buf_end, &state);