APPLEMAIL-51 Add missing stop implementation.

Subprojects/PEPUpdatesXPCService/PEPUpdatesXPCService/PEPUpdateDownloadXPCService/PEPUpdateDownloadXPCService/XPCService/PEPUpdateDownloadXPCService.m

@@ -36,6 +36,10 @@
     NSAssert(opened, @"Fail to open 'p≡p updates.app'.");
 }
 
+- (void)stop {
+    self.listener = nil;
+}
+
 // MARK: - Private
 
 - (BOOL)startPEPUpdatesApp {