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/ctx.cpp

@@ -353,7 +353,15 @@ zmq::endpoint_t zmq::ctx_t::find_endpoint (const char *addr_)
      return endpoint;
 }
 
-void zmq::ctx_t::monitor_event (zmq::socket_base_t *socket_, int event_, va_list args_)
+void zmq::ctx_t::monitor_event (zmq::socket_base_t *socket_, int event_, ...)
+{
+    va_list args;
+    va_start (event_, args);
+    va_monitor_event (socket_, event_, args);
+    va_end (args);
+}
+
+void zmq::ctx_t::va_monitor_event (zmq::socket_base_t *socket_, int event_, va_list args_)
 {
     if (monitor_fn != NULL) {
         zmq_event_data_t data;

src/ctx.hpp

@@ -97,7 +97,8 @@ namespace zmq
 
         // Monitoring specific
         int monitor (zmq_monitor_fn *monitor_);
-        void monitor_event (zmq::socket_base_t *socket_, int event_, va_list args_);
+        void monitor_event (zmq::socket_base_t *socket_, int event_, ...);
+        void va_monitor_event (zmq::socket_base_t *socket_, int event_, va_list args_);
 
         enum {
             term_tid = 0,

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)

src/session_base.hpp

@@ -24,6 +24,7 @@
 #define __ZMQ_SESSION_BASE_HPP_INCLUDED__
 
 #include <string>
+#include <stdarg.h>
 
 #include "own.hpp"
 #include "io_object.hpp"
@@ -67,6 +68,7 @@ namespace zmq
         void terminated (zmq::pipe_t *pipe_);
 
         void monitor_event (int event_, ...);
+        void va_monitor_event (int event_, va_list args);
 
     protected:
 

src/socket_base.cpp

@@ -1001,6 +1001,11 @@ void zmq::socket_base_t::monitor_event (int event_, ...)
 {
     va_list args;
     va_start (args, event_);
-    get_ctx ()->monitor_event (this, event_, args);
+    va_monitor_event(event, args);
     va_end (args);
 }
+
+void zmq::socket_base_t::monitor_event (int event_, va_list args)
+{
+    get_ctx ()->va_monitor_event (this, event_, args);
+}

src/socket_base.hpp

@@ -25,6 +25,7 @@
 
 #include <string>
 #include <map>
+#include <stdarg.h>
 
 #include "own.hpp"
 #include "array.hpp"
@@ -102,6 +103,7 @@ namespace zmq
         void unlock();
 
         void monitor_event (int event_, ...);
+        void va_monitor_event (int event_, va_list args);
 
     protected: