Don't crash if there's no post handler

src/Loop.h

@@ -55,8 +55,10 @@ private:
     static void postCb(us_loop *loop) {
         LoopData *loopData = (LoopData *) us_loop_ext(loop);
 
+        if (loopData->postHandler) {
             loopData->postHandler((Loop *) loop);
         }
+    }
 
     Loop() = delete;