bink: check vst->index_entries before using it

This fixes a NULL pointer dereference if vst->duration is 0.

The problem was introduced in commit 0588acaf.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/bink.c

@@ -194,7 +194,10 @@ static int read_header(AVFormatContext *s)
             return ret;
     }
 
+    if (vst->index_entries)
         avio_seek(pb, vst->index_entries[0].pos, SEEK_SET);
+    else
+        avio_skip(pb, 4);
 
     bink->current_track = -1;
     return 0;