mov: reset dref_count on realloc to keep values consistent.

This fixes a potential crash.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -401,6 +401,7 @@ static int mov_read_dref(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     if (entries >= UINT_MAX / sizeof(*sc->drefs))
         return AVERROR_INVALIDDATA;
     av_free(sc->drefs);
+    sc->drefs_count = 0;
     sc->drefs = av_mallocz(entries * sizeof(*sc->drefs));
     if (!sc->drefs)
         return AVERROR(ENOMEM);