JAVA-793: Fixed race condition in DBTCPConnector.authenticate by putting in a call to checkMaster prior to getting a DBPort to authenticate on.

Author: jyemin

src/main/com/mongodb/DBTCPConnector.java

@@ -614,6 +614,7 @@ public boolean isOpen(){
 
     @Override
     public CommandResult authenticate(MongoCredential credentials) {
+        checkMaster(false, true);
         final DBPort port = _myPort.get(false, ReadPreference.primaryPreferred(), null);
 
         try {