avformat/jacosubdec: Check for min in t overflow in get_shift()

Fixes: negation of -2147483648 cannot be represented in type 'int'; cast to an unsigned type to negate this value to itself
Fixes: 34651/clusterfuzz-testcase-minimized-ffmpeg_dem_JACOSUB_fuzzer-5157941012463616

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

@@ -134,6 +134,9 @@ static int get_shift(int timeres, const char *buf)
     int n = sscanf(buf, "%d"SSEP"%d"SSEP"%d"SSEP"%d", &a, &b, &c, &d);
 #undef SSEP
 
+    if (a == INT_MIN)
+        return 0;
+
     if (*buf == '-' || a < 0) {
         sign = -1;
         a = FFABS(a);