Fix several crashes in MeetMeAdmin

This change ensures that MeetMeAdmin commands requiring a user actually
get a user and fixes another issue where an extra dereference could
occur for a last-entered user being ejected if a user identifier was
also provided.

(closes issue ASTERISK-21907)
Reported by: Alex Epshteyn
Review: https://reviewboard.asterisk.org/r/2844/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@399033 f38db490-d61c-443f-a65b-d21fe96a405b