avformat/mov: Fix signed integer overflows with total_size

Signed integer overflow is undefined behavior.
Detected with clang and -fsanitize=signed-integer-overflow
Signed-off-by: Vitaly Buka <vitalybuka@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -5572,7 +5572,7 @@ static int mov_read_default(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 
     if (atom.size < 0)
         atom.size = INT64_MAX;
-    while (total_size + 8 <= atom.size && !avio_feof(pb)) {
+    while (total_size <= atom.size - 8 && !avio_feof(pb)) {
         int (*parse)(MOVContext*, AVIOContext*, MOVAtom) = NULL;
         a.size = atom.size;
         a.type=0;