avformat/mov: Check samplesize and offset to avoid integer overflow

Fixes: signed integer overflow: 9223372036854775584 + 536870912 cannot be represented in type 'long'
Fixes: 55844/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-510613920664780

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 53c1f5c2e2)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -4192,6 +4192,13 @@ static void mov_build_index(MOVContext *mov, AVStream *st)
                 if (keyframe)
                     distance = 0;
                 sample_size = sc->stsz_sample_size > 0 ? sc->stsz_sample_size : sc->sample_sizes[current_sample];
+                if (current_offset > INT64_MAX - sample_size) {
+                    av_log(mov->fc, AV_LOG_ERROR, "Current offset %"PRId64" or sample size %u is too large\n",
+                           current_offset,
+                           sample_size);
+                    return;
+                }
+
                 if (sc->pseudo_stream_id == -1 ||
                    sc->stsc_data[stsc_index].id - 1 == sc->pseudo_stream_id) {
                     AVIndexEntry *e;