fixes crash in moh when cachertclasses flag is used

The result for moh_register was not verified to guarantee
the mohclass as added to the container.


(closes issue #16993)
Reported by: dmitri
Patches:
      res_musiconhold_rtclass2.patch uploaded by dmitri (license 1001)
      moh_crash2.diff uploaded by dvossel (license 671)
Tested by: dmitri




git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@269271 65c4cc65-6c06-0410-ace0-fbb531ad65f3