Provide callback with closeNext

[clehner]

I was getting an error about a missing callback:

TypeError: cb is not a function
    at /usr/local/src/ssb-blobs/node_modules/multiblob/node_modules/pull-file/index.js:111:9
    at FSReqWrap.oncomplete (fs.js:82:15)

This happened when I browse to http://localhost:7718/%25A%2FQL%2BT42fBU7E%2FyzQuwKZhSZ8ytd9mSMeoGRla9DDfY%3D.sha256/blob/master/helloworld.html while running git-ssb-web

I added some console.log(new Error().stack) lines to reveal more of the call stack:

    at close (/usr/local/src/ssb-blobs/node_modules/multiblob/node_modules/pull-file/index.js:92:28)
    at /usr/local/src/ssb-blobs/node_modules/multiblob/node_modules/pull-file/index.js:78:9
    at FSReqWrap.oncomplete (fs.js:82:15)

    at open (/usr/local/src/ssb-blobs/node_modules/multiblob/node_modules/pull-file/index.js:73:21)
    at source (/usr/local/src/ssb-blobs/node_modules/multiblob/node_modules/pull-file/index.js:131:7)
    at next (/usr/local/src/pull-stream/sinks/drain.js:16:11)
    at sink (/usr/local/src/pull-stream/sinks/drain.js:37:9)
    at pull (/usr/local/src/pull-stream/pull.js:24:14)
    at Object.u.readObjectString (/usr/local/src/git-ssb-web/lib/util.js:115:3)
    at /usr/local/src/git-ssb-web/index.js:498:7
    at Array.<anonymous> (/usr/local/src/git-ssb-web/lib/util.js:34:5)
    at next (/usr/local/src/pull-cat/index.js:29:19)
    at Array.<anonymous> (/usr/local/src/pull-cat/index.js:44:7)

I'm not entirely sure what's going on, but I got the idea that when closeNext gets set, _cb is supposed to be set too. So adding this dummy callback function fixes the error that I was seeing.