avformat/mxfdec: Check first case of offset_temp computation for overflow

This is kind of ugly
Fixes: signed integer overflow: 255 * 1157565362826411919 cannot be represented in type 'long'
Fixes: 67313/clusterfuzz-testcase-minimized-ffmpeg_dem_MXF_fuzzer-6250434245230592

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 d6ed6f6e8d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mxfdec.c

@@ -1895,9 +1895,13 @@ static int mxf_edit_unit_absolute_offset(MXFContext *mxf, MXFIndexTable *index_t
         if (edit_unit < s->index_start_position + s->index_duration) {
             int64_t index = edit_unit - s->index_start_position;
 
-            if (s->edit_unit_byte_count)
+            if (s->edit_unit_byte_count) {
+                if (index > INT64_MAX / s->edit_unit_byte_count ||
+                    s->edit_unit_byte_count * index > INT64_MAX - offset_temp)
+                    return AVERROR_INVALIDDATA;
+
                 offset_temp += s->edit_unit_byte_count * index;
-            else {
+            } else {
                 if (s->nb_index_entries == 2 * s->index_duration + 1)
                     index *= 2;     /* Avid index */