chat: fix segfault when domain is not provided
chat: fix segfault when domain is not provided

Running xmppipe in chat mode without a full jid:

    xmppipe --chat --output foo

Caused a segfault when a NULL domain was passed to
xmpp_stanza_set_attribute(). The libstrophe functions do not check for
NULL and so crash calling strlen(NULL).

Set the jid's domain from the user's username. If the user's jid is
user@example.com:

    # expanded to foo@example.com
    xmppipe --chat --output foo

msantos/xmppipe,msantos/xmppipe