transfers: Fix bug setting both BLINDTRANSFER and ATTENDEDTRANSFER

The ast_bridge_set_transfer_variables function is supposed to wipe whichever
variable isn't being set. Instead it was setting both to the new value.  Oops.

(issue AFS-24)
........

Merged revisions 403957 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403958 65c4cc65-6c06-0410-ace0-fbb531ad65f3