Timer avoid deadlock in timerStopped()

No code actually uses this hook, but lets make
sure nothing bad would happen.

src/misc/timer.cpp

@@ -177,6 +177,7 @@ void Timer::schedulePeriodic(
         }
 
         if(!alive) {
+            xx.unlock();
             timerCallback->timerStopped();
             return;
         }