Fix overflow; "Timer: gettimeofday is broken"

The rounding misbehaved when the tv_usec calculation overflows.

Fixes issue TigerVNC#1782.

common/rfb/Timer.cxx

@@ -54,7 +54,10 @@ inline static timeval addMillis(timeval inTime, int millis) {
 }
 
 inline static int diffTimeMillis(timeval later, timeval earlier) {
-  return ((later.tv_sec - earlier.tv_sec) * 1000) + ((later.tv_usec - earlier.tv_usec + 999) / 1000);
+  long udiff;
+  udiff = ((later.tv_sec - earlier.tv_sec) * 1000000) +
+          (later.tv_usec - earlier.tv_usec);
+  return (udiff + 999) / 1000;
 }
 
 std::list<Timer*> Timer::pending;