res_pjsip_refer: Make safely get the context for a blind transfer.

res_pjsip_refer: Make safely get the context for a blind transfer.

Made safely get the TRANSFER_CONTEXT channel value while the channel is
locked in refer_incoming_attended_request() and
refer_incoming_blind_request().  The pointer returned by
pbx_builtin_getvar_helper() is only valid while the channel is locked.
........

Merged revisions 432594 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@432595 65c4cc65-6c06-0410-ace0-fbb531ad65f3