Fix a bug in timer loop

Silly logic bug introduced in dda7de17e7

Timer thread, when msec = 0, instead of going
to sleep, calls check_time() in an endless loop.

Spotted and reported by snino64 due to abnormally
high CPU usage.

No functional change.

src/thread.cpp

@@ -76,9 +76,10 @@ void TimerThread::idle_loop() {
   while (!do_exit)
   {
       mutex.lock();
-      while (!msec && !do_exit)
-          sleepCondition.wait_for(mutex, msec ? msec : INT_MAX);
+      do sleepCondition.wait_for(mutex, msec ? msec : INT_MAX);
+      while (!msec && !do_exit); // Don't allow wakeups when msec = 0
       mutex.unlock();
+
       check_time();
   }
 }