avformat/mov: Check a.size before computing next_root_atom

Fixes: signed integer overflow: 64 + 9223372036854775799 cannot be represented in type 'long'
Fixes: 27563/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-6244650163372032

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 8c9a5a0fe9)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -7046,7 +7046,7 @@ static int mov_read_default(MOVContext *c, AVIOContext *pb, MOVAtom atom)
                 c->atom_depth --;
                 return err;
             }
-            if (c->found_moov && c->found_mdat &&
+            if (c->found_moov && c->found_mdat && a.size <= INT64_MAX - start_pos &&
                 ((!(pb->seekable & AVIO_SEEKABLE_NORMAL) || c->fc->flags & AVFMT_FLAG_IGNIDX || c->frag_index.complete) ||
                  start_pos + a.size == avio_size(pb))) {
                 if (!(pb->seekable & AVIO_SEEKABLE_NORMAL) || c->fc->flags & AVFMT_FLAG_IGNIDX || c->frag_index.complete)