this can resolve  #5

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@4130 43c1dc8c-776e-4df2-8ef1-6e829299be21
Set initial value for extension_exists

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@4131 43c1dc8c-776e-4df2-8ef1-6e829299be21
Check participant returned from sccp_conference_participant_findByPBXChannel before using it

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@4132 43c1dc8c-776e-4df2-8ef1-6e829299be21