res_parking: Fix bug where reloading immediately wipes new parkpos extensions

(closes issue ASTERISK-22631)
Reported by: Kevin Harwell
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Merged revisions 401158 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@401159 f38db490-d61c-443f-a65b-d21fe96a405b