EJBCLIENT-243 Make sure only the first result is taken into account.

When proceedAsynchronously() the state can still be SENT, and then
if the async method throws an exception from another thread the result
producer can be changed to the exception producer, resulting in incorrect
behaviour

src/main/java/org/jboss/ejb/client/EJBClientInvocationContext.java

@@ -637,7 +637,7 @@ public void discardResult() throws IllegalStateException {
     void resultReady(EJBReceiverInvocationContext.ResultProducer resultProducer) {
         Assert.checkNotNullParam("resultProducer", resultProducer);
         synchronized (lock) {
-            if (state.isWaiting()) {
+            if (state.isWaiting() && this.resultProducer == null) {
                 this.resultProducer = resultProducer;
                 if (state == State.WAITING) {
                     transition(State.READY);
@@ -649,7 +649,6 @@ void resultReady(EJBReceiverInvocationContext.ResultProducer resultProducer) {
         }
         // for whatever reason, we don't care
         resultProducer.discardResult();
-        return;
     }
 
     /**