avformat/mov: Correct opus-in-mp4 pre-skip to be uint16_t versus int16_t.

This field is a uint16_t, see docs:
http://opus-codec.org/docs/opus_in_isobmff.html#4.3.2Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -6614,7 +6614,7 @@ static int mov_read_dops(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     const int OPUS_SEEK_PREROLL_MS = 80;
     AVStream *st;
     size_t size;
-    int16_t pre_skip;
+    uint16_t pre_skip;
 
     if (c->fc->nb_streams < 1)
         return 0;