Protect against overflow in timeval_minus_msec.

This would cause an infinite loop when the clock was stepped.

util.c

@@ -79,6 +79,12 @@ timeval_minus(struct timeval *d,
 int
 timeval_minus_msec(const struct timeval *s1, const struct timeval *s2)
 {
+    /* Avoid overflow.  This may happen if the clock is changed */
+    if(s1->tv_sec - s2->tv_sec > 2000000)
+        return 2000000000;
+    else if(s1->tv_sec - s2->tv_sec < -2000000)
+        return -2000000000;
+
     return (s1->tv_sec - s2->tv_sec) * 1000 +
         (s1->tv_usec - s2->tv_usec) / 1000;
 }