Don't create CFSocket unnecessarily when closing wxSocket

DoClose() shouldn't do anything if CFSocket hadn't been created at all
(as will be the case for blocking sockets) instead of creating it only
to destroy it immediately afterwards.

src/osx/core/sockosx.cpp

@@ -63,6 +63,12 @@ public:
 private:
     virtual void DoClose() wxOVERRIDE
     {
+        // No need to do anything if we had never created the underlying
+        // socket: this avoids creating it from Uninstall_Callback() completely
+        // unnecessarily.
+        if ( !m_socket )
+            return;
+
         wxSocketManager * const manager = wxSocketManager::Get();
         if ( manager )
         {