avformat/mov: Check size before subtraction

Fixes: signed integer overflow: -9223372036854775808 - 8 cannot be represented in type 'long'
Fixes: 43542/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-5237670148702208

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -7329,6 +7329,8 @@ static int mov_read_default(MOVContext *c, AVIOContext *pb, MOVAtom atom)
         if (a.size == 0) {
             a.size = atom.size - total_size + 8;
         }
+        if (a.size < 0)
+            break;
         a.size -= 8;
         if (a.size < 0)
             break;