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/
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Merged revisions 399033 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 399034 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 399035 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@399036 65c4cc65-6c06-0410-ace0-fbb531ad65f3