MUC: hoist JID decoding up to main <presence> handler
MUC: hoist JID decoding up to main <presence> handler

The <presence type='error'/> sub-handler actually didn't use the JID
components at all, it just parsed it for fun. And the other sub-handler
only uses the resource. It's neater to pull extracting that into the top
handler: it'll save us needing a 'goto' for cleanup.

noonien-d/wocky,freedesktop-unofficial-mirror/wocky,noonien-d/wocky,noonien-d/wocky,freedesktop-unofficial-mirror/wocky,freedesktop-unofficial-mirror/wocky