Filesystem.Monitor: Improved robustness.

It seems ADT.Heap()->peek() sometimes can index the NULL value
(cf [bug 7644]). The most likely cause is that the last element
of the heap was removed by a different thread during the call.
Inhibit propagation of the error to the caller.

Work around for [bug 7644].

lib/modules/Filesystem.pmod/Monitor.pmod/basic.pike

@@ -1490,6 +1490,11 @@ int check(int|void max_wait, int|void max_cnt,
     int cnt;
     int t = time();
     if (sizeof(monitor_queue)) {
+      // NB: peek() can apparently in some circumstances throw errors.
+      //     cf [bug 7644]. The likely cause being that a different
+      //     thread removed the last element during the call. Make
+      //     sure not to propagate the error to the caller.
+      mixed err = catch {
 	  Monitor m;
 	  while ((m = monitor_queue->peek()) && (m->next_poll <= t)) {
 	    cnt += check_monitor(m);
@@ -1504,6 +1509,10 @@ int check(int|void max_wait, int|void max_cnt,
 	  } else {
 	    scan_cnt--;
 	  }
+	};
+      if (err) {
+	master()->handle_error(err);
+      }
     }
     if (cnt || !scan_wait || !scan_cnt) {
       if (ret_stats) {