fix(xep): Fix failure with multiple SCRAM negotiators and SASL2

packages/moxxmpp/lib/src/rfcs/rfc_6120/sasl/plain.dart

@@ -96,7 +96,9 @@ class SaslPlainNegotiator extends Sasl2AuthenticationNegotiator {
 
   @override
   Future<Result<bool, NegotiatorError>> onSasl2Success(XMLNode response) async {
-    state = NegotiatorState.done;
+    if (pickedForSasl2) {
+      state = NegotiatorState.done;
+    }
     return const Result(true);
   }
 

packages/moxxmpp/lib/src/rfcs/rfc_6120/sasl/scram.dart

@@ -363,6 +363,11 @@ class SaslScramNegotiator extends Sasl2AuthenticationNegotiator {
 
   @override
   Future<Result<bool, NegotiatorError>> onSasl2Success(XMLNode response) async {
+    // Don't do anything if we have not been picked for SASL2.
+    if (!pickedForSasl2) {
+      return const Result(true);
+    }
+
     // When we're done with SASL2, check the additional data to verify the server
     // signature.
     state = NegotiatorState.done;