res_rtp_asterisk: Ensure that the base and mapped address for candidates is present in SDP.

This change fixes an issue where ICE candidates put into the SDP did not contain
the 'raddr' and 'rport' information for server reflexive and relay candidates.

#SIPit31


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@424151 f38db490-d61c-443f-a65b-d21fe96a405b