Check return of url_fseek in mpeg timestamp reading code.

Patch by Joakim elupus ecce se

Originally committed as revision 13527 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mpeg.c

@@ -557,7 +557,9 @@ static int64_t mpegps_read_dts(AVFormatContext *s, int stream_index,
 #ifdef DEBUG_SEEK
     printf("read_dts: pos=0x%"PRIx64" next=%d -> ", pos, find_next);
 #endif
-    url_fseek(s->pb, pos, SEEK_SET);
+    if (url_fseek(s->pb, pos, SEEK_SET) < 0)
+        return AV_NOPTS_VALUE;
+
     for(;;) {
         len = mpegps_read_pes_header(s, &pos, &startcode, &pts, &dts);
         if (len < 0) {