mov: avoid a memleak when multiple stss boxes are present

CC: libav-stable@libav.org
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind

libavformat/mov.c

@@ -1819,6 +1819,7 @@ static int mov_read_stss(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     }
     if (entries >= UINT_MAX / sizeof(int))
         return AVERROR_INVALIDDATA;
+    av_freep(&sc->keyframes);
     sc->keyframes = av_malloc(entries * sizeof(int));
     if (!sc->keyframes)
         return AVERROR(ENOMEM);