ARI: Fix crash when POST /playback/{id}/control does not have an operation parameter.

(closes issue ASTERISK-22680)
Reported by: John Bigelow
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Merged revisions 401107 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@401108 65c4cc65-6c06-0410-ace0-fbb531ad65f3