provide callback for muxrpc.close

[mixmix]

Problem: in nodejs mobile we were getting an app crash on using an invite:

      else if (err) throw clarify(err, 'no callback provided for muxrpc close')
                    ^

Error: no callback provided for muxrpc close
    at /data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:45138:27
    at /data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:15455:36
    at Array.forEach (<anonymous>)
    at PacketStream._maybedone (/data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:15455:18)
    at PacketStream.destroy (/data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:15439:8)
  Error: unexpected end of parent stream
    at PacketStream.destroy (/data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:15414:7)
    at PacketStream.write (/data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:15476:41)
    at /data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:108957:17
    at /data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:120201:24
    at /data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:193644:11
    at pscDecodeRead (/data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:136990:25)
    at next (/data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:193641:9)
    at /data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:193643:41
    at Object.pscDecodeHead [as cb] (/data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:136998:18)
    at drain (/data/user/0/io.ahau.whakapapa/files/www/nodejs-project/index.js:130769:14)

Tracked this down into muxrpc and I think what's happening is a race condition around ps and ws closing. We were only seeing this crash when Ben (in Colombia) was trying to use an invite to my pub in New Zealand 😢 (no such problems in NZ peer -> NZ pub)

ANYWAY the throw is happening because there is no cb being provided on close. So I've provided a callback ...