Fix for fs.appendFile failure when a callback isn't passed and an is error returned

[itaisatati]

Documentation for 'fs.appendFile' marks the 'callback' argument as optional, but the code would fail if not passed a callback function and an error has occurred.

The code defaulted to 'null' which was later called as a function in the case of an error.

Fixed to use (the already defined) 'makeCallback' function.

[bnoordhuis]

LGTM if it works as advertised. However:

1. Can you add a regression test or two?
2. Can you rework the commit message as described here?
3. Last but not least, can you sign the CLA?

Thanks!

[itaisatati]

Sorry about that. My fault for not searching the notes about contributing.
Will fix the commit message and sign the CLA now and take care of the test tomorrow.

[Nodejs-Jenkins]

Can one of the admins verify this patch?

[tjfontaine]

This is fixed in current releases.