fix: prevent delete from sending multiple args in callback

[jacobheun]

This fixes an issue in node10 where unlink could call the callback as callback(err, undefined). This resulted in test suite failures for ipfs-repo as it was attempting to use async/waterfall.

This PR simply enforces the callback only ever returning an error, if one exists.

The following chain would cause the callback being sent to datastore.has to undefined.

        waterfall([
          (cb) => repo.datastore.delete(b, cb),
          (cb) => repo.datastore.has(b, cb)
        ], (err, exists) => {
          expect(err).to.not.exist()
          expect(exists).to.equal(false)
          done()
        })

This is related to the following issues, but is not the core cause, it is just causing test suite failures in node v10.
relates to ipfs/js-ipfs#1347
relates to ipfs/js-ipfs-repo#167

[jacobheun]

CI has been resolved, this should be good to get merged.

[jacobheun]

FYI, the underlying node issue has been filed here: nodejs/node#20872

[jacobheun]

@dignifiedquire @diasdavid can we get this released? This update is being used and is needed for the js-ipfs node 10 fixes.

[daviddias]

you got it! Released with 0.5.0