Since adding the AST_CONTROL_SRCUPDATE frame type,

Since adding the AST_CONTROL_SRCUPDATE frame type,
there are places where ast_rtp_new_source may be called
where the tech_pvt of a channel may not yet have an
rtp structure allocated. This caused a crash in chan_skinny,
which was fixed earlier, but now the same crash has been
reported against chan_h323 as well. It seems that the best
solution is to modify ast_rtp_new_source to not attempt to
set the marker bit if the rtp structure passed in is NULL.

This change to ast_rtp_new_source also allows the removal
of what is now a redundant pointer check from chan_skinny.

(closes issue #13247)
Reported by: pj

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@136062 65c4cc65-6c06-0410-ace0-fbb531ad65f3