avformat/jacosubdec: Use 64bit intermediate for start/end timestamp shift

Fixes: signed integer overflow: -1957694447 + -1620425806 cannot be represented in type 'int'
Fixes: 30207/clusterfuzz-testcase-minimized-ffmpeg_dem_JACOSUB_fuzzer-5050791771635712

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

libavformat/jacosubdec.c

@@ -125,8 +125,8 @@ static const char *read_ts(JACOsubContext *jacosub, const char *buf,
     return NULL;
 
 shift_and_ret:
-    ts_start64  = (ts_start + jacosub->shift) * 100LL / jacosub->timeres;
-    ts_end64    = (ts_end   + jacosub->shift) * 100LL / jacosub->timeres;
+    ts_start64  = (ts_start + (int64_t)jacosub->shift) * 100LL / jacosub->timeres;
+    ts_end64    = (ts_end   + (int64_t)jacosub->shift) * 100LL / jacosub->timeres;
     *start    = ts_start64;
     *duration = ts_end64 - ts_start64;
     return buf + len;