sipcall: fix nullptr access in remote sdp handling

sipvoiplink code may set remote sdp to nullptr and this case was not
handled correctly later.
This patch fixes that by not using the nullptr to set ICE call transport.
Also moved the code to use the remote sdp, duplicated, into sipcall
for factorization.

Change-Id: I6dcfd3bebed3fd754c91dd7f2cab7ca110d3de09