avformat/jacosubdec: Fix overflow in get_shift()

Fixes: 50993/clusterfuzz-testcase-minimized-ffmpeg_dem_JACOSUB_fuzzer-6722544461283328
Fixes: signed integer overflow: 48214448 * 60 cannot be represented in type 'int'

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

@@ -144,7 +144,7 @@ static int get_shift(int timeres, const char *buf)
     ret = 0;
     switch (n) {
     case 4:
-        ret = sign * (((int64_t)a*3600 + b*60 + c) * timeres + d);
+        ret = sign * (((int64_t)a*3600 + (int64_t)b*60 + c) * timeres + d);
         break;
     case 3:
         ret = sign * ((         (int64_t)a*60 + b) * timeres + c);