add support for RTP/SAVPF to sofia stack
mod_sofia: Follow-up fix for FS-5009 to avoid NULL-ptr dereference

Commit f913670e changed switch_core_session_sprintf() args
from sip->sip_to to sip->sip_from, but didn't update the enclosing if()
clause protecting the code from dereferencing NULL pointers.

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>
FS-5018 --resolve