mov: fix leaking memory with multiple drefs.

Instead of allocating over the original, free first. MOVStreamContext
is zero initialized so no double free will occur. Same style as other
fixes for the same problem in this file.
Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/mov.c

@@ -387,6 +387,7 @@ static int mov_read_dref(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     entries = avio_rb32(pb);
     if (entries >= UINT_MAX / sizeof(*sc->drefs))
         return AVERROR_INVALIDDATA;
+    av_free(sc->drefs);
     sc->drefs = av_mallocz(entries * sizeof(*sc->drefs));
     if (!sc->drefs)
         return AVERROR(ENOMEM);