Initialize latestContacts to empty dict (#1207)

Seeing intermittent segfaults on [latestContacts allKeys].
This could be because latestContacts is initialized async.

//FREEBIE

Signal/src/contact/ContactsManager.m

@@ -20,6 +20,7 @@ typedef BOOL (^ContactSearchBlock)(id, NSUInteger, BOOL *);
     if (self) {
         life                         = [TOCCancelTokenSource new];
         observableContactsController = [ObservableValueController observableValueControllerWithInitialValue:nil];
+        latestContactsById           = @{};
     }
     return self;
 }