res_rtp_asterisk: Return the length of data written when sending via ICE instead of 0.

ASTERISK-23834 #close
Reported by: Richard Kenner
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Merged revisions 417141 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 417142 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@417143 65c4cc65-6c06-0410-ace0-fbb531ad65f3