notifications: leave dispatch group when timeout occurs

This patch stops tasks for the dispatch group when the notification
extension times out, to prevent potential memory leaks.

Change-Id: I1b69500d4026a5244d49c728887a1c4fd55a3fd2
diff --git a/Ring/jamiNotificationExtension/NotificationService.swift b/Ring/jamiNotificationExtension/NotificationService.swift
index a7c9b88..917810e 100644
--- a/Ring/jamiNotificationExtension/NotificationService.swift
+++ b/Ring/jamiNotificationExtension/NotificationService.swift
@@ -156,6 +156,10 @@
     }
 
     override func serviceExtensionTimeWillExpire() {
+        if !self.tasksCompleted {
+            self.tasksCompleted = true
+            self.tasksGroup.leave()
+        }
         finish()
     }