avformat/vpk: check that samples_per_block is > 0

Fixes floating point exception.

libavformat/vpk.c

@@ -121,7 +121,10 @@ static int vpk_read_seek(AVFormatContext *s, int stream_index,
     int64_t ret = 0;
 
     samples_per_block = av_get_audio_frame_duration2(par, par->block_align);
-    timestamp /= samples_per_block;
+    if (samples_per_block > 0)
+        timestamp /= samples_per_block;
+    else
+        return -1;
     ret = avio_seek(s->pb, vpk->data_start + timestamp * par->block_align, SEEK_SET);
     if (ret < 0)
         return ret;