Properly initializes UDP message listener.

This properly initializes the ServerPool reference to NULL in the UDP
Listener class, following 8532584, allowing its current state to be
properly detected, and allowing it to be brought up and down
successfully.

Fixes #11037

mythtv/libs/libmythui/mythudplistener.cpp

@@ -11,7 +11,8 @@
 
 #define LOC QString("UDPListener: ")
 
-MythUDPListener::MythUDPListener()
+MythUDPListener::MythUDPListener() :
+    m_socketPool(NULL)
 {
     Enable();
 }