Fix monitor_event() to work at all.

There are three versions of monitor_event(), all taking
variadic arguments. The original code just has the first one
creating a va_list and passing that va_list variadically to
the second one... which creates a new va_list and passes it
variadically to the third one... and of course everything
blows up when we try to pull a non-va_list argument off the
stack.

The correct approach matches the C standard library's use
of printf/vprintf, scanf/vscanf, and so on. Once you make
a va_list, you must pass it only to functions which expect
a va_list parameter.

src/session_base.cpp

@@ -290,10 +290,15 @@ void zmq::session_base_t::monitor_event (int event_, ...)
 {
     va_list args;
     va_start (args, event_);
-    socket->monitor_event (event_, args);
+    va_monitor_event (event_, args);
     va_end (args);
 }
 
+void zmq::session_base_t::va_monitor_event (int event_, va_list args)
+{
+    socket->va_monitor_event (event_, args);
+}
+
 void zmq::session_base_t::process_plug ()
 {
     if (connect)