avformat/mpegts: Warn if ffio_ensure_seekback() failed

Should silence CID1257005
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mpegts.c

@@ -2454,7 +2454,8 @@ static int mpegts_read_header(AVFormatContext *s)
     int len;
     int64_t pos, probesize = s->probesize ? s->probesize : s->probesize2;
 
-    ffio_ensure_seekback(pb, probesize);
+    if (ffio_ensure_seekback(pb, probesize) < 0)
+        av_log(s, AV_LOG_WARNING, "Failed to allocate buffers for seekback\n");
 
     /* read the first 8192 bytes to get packet size */
     pos = avio_tell(pb);