Fix throw error, if send presence to a not exist room.

When user send  presence to a not exist room will cause an error.
`[TypeError: Cannot call method 'getChild' of undefined]`

Then stanza is like this:
```
{ name: 'presence',
  parent: null,
  attrs:
   { from: 'all@muc.frank-test.octochat.local/xeodou',
     to: 'xeodou@frank-test.octochat.local/xeodou',
     type: 'error',
     'xml:lang': '',
     'xmlns:stream': 'http://etherx.jabber.org/streams' },
  children:
   [ { name: 'x', parent: [Circular], attrs: [Object], children: [] },
     { name: 'error',
       parent: [Circular],
       attrs: [Object],
       children: [Object] } ] }
```
More detail you check this gist https://gist.github.com/xeodou/74003dcafdbafcad2dab