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

(closes issue ASTERISK-22680)
Reported by: John Bigelow
........

Merged revisions 401107 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@401108 f38db490-d61c-443f-a65b-d21fe96a405b