Don't set the full command string until after verifying that there is not

Don't set the full command string until after verifying that there is not
another CLI command with the same command text registered.  This prevents
a crash if someone accidentally calls ast_cli_register()  on the same CLI
command data twice.  This also fixes a small bug where the helpers list
would get unlocked without being locked if building the full command failed.
(closes issue #10858, reported by jamesgolovich, patched by me)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@84169 65c4cc65-6c06-0410-ace0-fbb531ad65f3