nsvdec: Check return value of avio_seek and avoid modifying state if it fails

Signed-off-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>

libavformat/nsvdec.c

@@ -720,7 +720,9 @@ static int nsv_read_seek(AVFormatContext *s, int stream_index, int64_t timestamp
     if(index < 0)
         return -1;
 
-    avio_seek(s->pb, st->index_entries[index].pos, SEEK_SET);
+    if (avio_seek(s->pb, st->index_entries[index].pos, SEEK_SET) < 0)
+        return -1;
+
     nst->frame_offset = st->index_entries[index].timestamp;
     nsv->state = NSV_UNSYNC;
     return 0;