chan_sip: Make direct media reinvites for T38 put Asterisk in the media path

Prior to this patch, Asterisk would incorrectly use the previous endpoint
addresses in SDP in spite of providing its own port. T38 is never meant to
be done through directmedia and Asterisk should always be in the media path
for these streams.

(closes issue ASTERISK-17273)
Reported by: Kevin Stewart

(closes issue ASTERISK-18706)
Reported by: Jeremy Kister

Review: https://reviewboard.asterisk.org/r/2853/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@399456 f38db490-d61c-443f-a65b-d21fe96a405b