Missed reference in null client for CL_PacketEvent

(cherry picked from commit c11082b)

codemp/null/null_client.cpp

@@ -41,7 +41,7 @@ void Key_WriteBindings( fileHandle_t f ) {
 void CL_Frame ( int msec ) {
 }
 
-void CL_PacketEvent( netadr_t from, msg_t *msg ) {
+void CL_PacketEvent( const netadr_t *from, msg_t *msg ) {
 }
 
 void CL_CharEvent( int key ) {