Merge pull request #2869 from evoskuil/master

Problem: fn pointers are not bool (vc++ warning C4550).

src/timers.cpp

@@ -50,7 +50,7 @@ bool zmq::timers_t::check_tag ()
 
 int zmq::timers_t::add (size_t interval_, timers_timer_fn handler_, void *arg_)
 {
-    if (!handler_) {
+    if (handler_ == NULL) {
         errno = EFAULT;
         return -1;
     }