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/
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Merged revisions 399456 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 399457 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 399458 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@399459 f38db490-d61c-443f-a65b-d21fe96a405b