Ensure that the cleanup method does not cause a crash due to accountType being nil.

As this should never be called anyway just be safe. Thanks to @mdamjanic7 for pointing out crashes caused by this.

Author: toto

Sources/OAuth2Client/NXOAuth2AccountStore.m

@@ -511,7 +511,9 @@ - (void)oauthClientDidLoseAccessToken:(NXOAuth2Client *)client;
     NSString *accountType;
     @synchronized (self.pendingOAuthClients) {
         accountType = [self accountTypeOfPendingOAuthClient:client];
-        [self.pendingOAuthClients removeObjectForKey:accountType];
+        if (accountType) {
+            [self.pendingOAuthClients removeObjectForKey:accountType];
+        }
     }
 }