mxfdec: fix typo in mxf_read_seek()

Check the number of index tables before using byte offset based seeking
instead of the index_tables pointer.

Found by Måns Rullgård <mans@mansr.com>.

libavformat/mxfdec.c

@@ -2210,7 +2210,7 @@ static int mxf_read_seek(AVFormatContext *s, int stream_index, int64_t sample_ti
     int ret;
     MXFIndexTable *t;
 
-    if (mxf->index_tables <= 0) {
+    if (mxf->nb_index_tables <= 0) {
     if (!s->bit_rate)
         return AVERROR_INVALIDDATA;
     if (sample_time < 0)