ARTEMIS-500 - possible index out of bounds after amqp sasl handshake

https://issues.apache.org/jira/browse/ARTEMIS-500

artemis-protocols/artemis-proton-plug/src/main/java/org/proton/plug/handler/impl/ProtonHandlerImpl.java

@@ -171,6 +171,10 @@ public class ProtonHandlerImpl extends ProtonInitializable implements ProtonHand
                try {
                   if (buffer.getByte(4) == 0x03) {
                      dispatchSASL();
+                     /*
+                     * there is a chance that if SASL Handshake has been carried out that the capacity may change.
+                     * */
+                     capacity = transport.capacity();
                   }
                }
                catch (Throwable ignored) {