Prevent segfault when messagebuffer is ''

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5025 43c1dc8c-776e-4df2-8ef1-6e829299be21
Added todo to sccp_rtp.c

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5026 43c1dc8c-776e-4df2-8ef1-6e829299be21