added sanity checking of poll timeout to prevent infinite wait

if any ticks are overdue (this whole thing will be revamped soon
to be simpler and more efficient, so this is a temporary solution).

overlay.c

@@ -235,6 +235,7 @@ int overlayServerMode()
 	DEBUGF("Waiting via poll() for up to %lldms", ms);
       TIMING_PAUSE();
       
+      if (ms>15000) ms=15000;
       r = poll(fds, fdcount, ms);
       
       TIMING_CHECK();