mov: avoid a memleak when multiple stss boxes are present

CC: libav-stable@libav.org
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
(cherry picked from commit 64f7575fbd)
Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit 577f1feb3f)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/mov.c

@@ -1598,6 +1598,7 @@ 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);