mxf: Do not use int to check the seek position

Overly large files are to be expected.
Reported-by: Jean-Baptiste Kempf <jb@videolan.org>

libavformat/mxfdec.c

@@ -2238,8 +2238,10 @@ static int mxf_read_seek(AVFormatContext *s, int stream_index, int64_t sample_ti
         sample_time = 0;
     seconds = av_rescale(sample_time, st->time_base.num, st->time_base.den);
 
-    if ((ret = avio_seek(s->pb, (s->bit_rate * seconds) >> 3, SEEK_SET)) < 0)
-        return ret;
+    seekpos = avio_seek(s->pb, (s->bit_rate * seconds) >> 3, SEEK_SET);
+    if (seekpos < 0)
+        return seekpos;
+
     ff_update_cur_dts(s, st, sample_time);
     mxf->current_edit_unit = sample_time;
     } else {