make 'module unload chan_skinny.so' actually work.

(closes issue #13524)
Reported by: wedhorn
Patches:
      unload.diff uploaded by wedhorn (license 30)
	  With small tweak by me to prevent a crash


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@143799 65c4cc65-6c06-0410-ace0-fbb531ad65f3