avformat/mov: Fix potential integer overflow in mov_read_keys

Actual allocation size is computed as (count + 1)*sizeof(meta_keys), so
we need to check that (count + 1) won't cause overflow.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -3278,7 +3278,7 @@ static int mov_read_keys(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 
     avio_skip(pb, 4);
     count = avio_rb32(pb);
-    if (count > UINT_MAX / sizeof(*c->meta_keys)) {
+    if (count > UINT_MAX / sizeof(*c->meta_keys) - 1) {
         av_log(c->fc, AV_LOG_ERROR,
                "The 'keys' atom with the invalid key count: %d\n", count);
         return AVERROR_INVALIDDATA;