res_clialiases: Fix crash when reloading and re-aliasing an alias that is in use.

The code assumed that unregistering the alias would always succeed while in
practice this is not actually true. A common case is the "reload" command itself.
If the cli_aliases.conf configuration file was changed and reload executed the
command would fail to unregister and ultimately point to freed memory.

The reload process now checks whether unregistering succeeded or not and if not
the old CLI alias is retained.

(closes issue ASTERISK-19773)
Reported by: Joel Vandal

(closes issue ASTERISK-22757)
Reported by: Gareth Blades
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Merged revisions 407205 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 407210 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@407213 f38db490-d61c-443f-a65b-d21fe96a405b