Fix a crash when res_xmpp is configured using a username without a domain.

(closes issue ASTERISK-21156)
Reported by: amsoft2001
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Merged revisions 382923 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@382924 f38db490-d61c-443f-a65b-d21fe96a405b