Fix: Prevent Indicate Ringing from cleaning out the called party name if already set correctly. Adapt sccp_channel_set_calledparty to make it possible to distinquish between leaving the name alone or cleaning it out. (Reported/Tested by Nikolay Vasilev)

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5676 43c1dc8c-776e-4df2-8ef1-6e829299be21