Enhancement: Check if conference->bridge exists before trying to join it. Issue reported by Asela Fernando

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5774 43c1dc8c-776e-4df2-8ef1-6e829299be21
Fix: refcount array length should be prime number.

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5775 43c1dc8c-776e-4df2-8ef1-6e829299be21
Enhancement: Check if conference->bridge was created succesfully during conference_create or return NULL, and cancel conference. Issue reported by Asela Fernando

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5776 43c1dc8c-776e-4df2-8ef1-6e829299be21