Disable recursive watchdog signal handler

If we are in the signal handler, we don't want to handle
the signal again.  In extreme cases, this can cause a stack overflow
and segfault Redis.

Fixes #1771

src/debug.c

@@ -931,7 +931,7 @@ void enableWatchdog(int period) {
         /* Watchdog was actually disabled, so we have to setup the signal
          * handler. */
         sigemptyset(&act.sa_mask);
-        act.sa_flags = SA_NODEFER | SA_ONSTACK | SA_SIGINFO;
+        act.sa_flags = SA_ONSTACK | SA_SIGINFO;
         act.sa_sigaction = watchdogSignalHandler;
         sigaction(SIGALRM, &act, NULL);
     }