avformat/jacosubdec: Factorize code in get_shift() a bit

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 6490b9aed6)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/jacosubdec.c

@@ -143,16 +143,12 @@ static int get_shift(int timeres, const char *buf)
 
     ret = 0;
     switch (n) {
-    case 4:
-        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);
-        break;
-    case 2:
-        ret = sign * ((                (int64_t)a) * timeres + b);
-        break;
+    case 1:                      a = 0;
+    case 2:        c = b; b = a; a = 0;
+    case 3: d = c; c = b; b = a; a = 0;
     }
+
+    ret = sign * (((int64_t)a*3600 + (int64_t)b*60 + c) * timeres + d);
     if ((int)ret != ret)
         ret = 0;