Fix: hangup when called without a non-existing sccp channel would unref the channel before setting channel tech to NULL, causing a segfault in ast_channel_destructor while free-ing channel tech. (Reported by Dave Burgess)

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5657 43c1dc8c-776e-4df2-8ef1-6e829299be21