sync.c: change ULONG_MAX to UINT32_MAX

The value returned is uint32_t, not unsigned long.  This creates a
warning when compiling on 64-bit machines.

sync.c

@@ -108,6 +108,6 @@ uint32_t next_sync(uint32_t position, uint32_t *length)
     } else {
         if (length)
             *length = 0L;
-        return ULONG_MAX;
+        return UINT32_MAX;
     }
 }