Fix initialization of dictionary

- Actually initialize the dictionary that stores refresh handles on a user
- Fix issue where refresh handle lifetime could be prolonged unnecessarily, which masked the above issue

Realm/RLMSyncSessionRefreshHandle.mm

@@ -248,8 +248,9 @@ - (void)_timerFired:(__unused NSTimer *)timer {
                            kRLMSyncAppIDKey: [RLMSyncManager sharedManager].appID,
                            };
 
+    __weak RLMSyncSessionRefreshHandle *weakSelf = self;
     RLMSyncCompletionBlock handler = ^(NSError *error, NSDictionary *json) {
-        [self _onRefreshCompletionWithError:error json:json];
+        [weakSelf _onRefreshCompletionWithError:error json:json];
     };
     [RLMNetworkClient postRequestToEndpoint:RLMServerEndpointAuth
                                      server:user.authenticationServer

Realm/RLMSyncUser.mm

@@ -72,6 +72,7 @@ + (RLMSyncUser *)currentUser {
 - (instancetype)initWithAuthServer:(nullable NSURL *)authServer {
     if (self = [super init]) {
         self.authenticationServer = authServer;
+        self.refreshHandles = [NSMutableDictionary dictionary];
         return self;
     }
     return nil;