avformat/jacosubdec: Use 64bit inside get_shift()

Fixes: signed integer overflow: 111111111 * 30 cannot be represented in type 'int'
Fixes: 26448/clusterfuzz-testcase-minimized-ffmpeg_dem_JACOSUB_fuzzer-5638440374501376

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

@@ -136,6 +136,7 @@ static int get_shift(int timeres, const char *buf)
 {
     int sign = 1;
     int a = 0, b = 0, c = 0, d = 0;
+    int64_t ret;
 #define SSEP "%*1[.:]"
     int n = sscanf(buf, "%d"SSEP"%d"SSEP"%d"SSEP"%d", &a, &b, &c, &d);
 #undef SSEP
@@ -145,13 +146,16 @@ static int get_shift(int timeres, const char *buf)
         a = FFABS(a);
     }
 
+    ret = 0;
     switch (n) {
-    case 4: return sign * ((a*3600 + b*60 + c) * timeres + d);
-    case 3: return sign * ((         a*60 + b) * timeres + c);
-    case 2: return sign * ((                a) * timeres + b);
+    case 4: ret = sign * (((int64_t)a*3600 + b*60 + c) * timeres + d);
+    case 3: ret = sign * ((         (int64_t)a*60 + b) * timeres + c);
+    case 2: ret = sign * ((                (int64_t)a) * timeres + b);
     }
+    if ((int)ret != ret)
+        ret = 0;
 
-    return 0;
+    return ret;
 }
 
 static int jacosub_read_header(AVFormatContext *s)