Fix XMPP nickname parsing

[maidl0ver]

Fixes parsing of XMPP nicknames containing / and/or @

Example with jid@srv.tld/nick/n@me:
Previous behaviour produces an empty string
New behaviour produces the intended nick/n@me

[selfhoster1312]

I'm actually not sure that @ is allowed in resource parts. See https://xmpp.org/rfcs/rfc4622.html

What client/server are you using that allows it? Can you find a reference for it to be allowed? If so we should open an issue in upstream https://github.com/xmppo/go-xmpp to add a helper function for that purpose.

[maidl0ver]

@selfhoster1312 I don't think the RFC you're referring to describes our case explicitly, here's a more concrete mention of what can go in the resource part: RFC7622 (3.4)

thus, for example, an XMPP address of the form <localpart@domainpart/foo/bar> does not identify a resource "bar" that exists below a resource "foo" in a hierarchy of resources associated with the entity "localpart@domainpart".

The '@' character is allowed in the resourcepart and is often used in the "handle" shown in XMPP chatrooms [XEP-0045]

[selfhoster1312]

Thanks for digging this high quality source! We would definitely accept this PR in the matterbridge community fork: https://github.com/matterbridge-org/matterbridge

Maybe they'll also be interested in upstream go-xmpp, but that's a story for another day :)