chan_sip: Fix theoretical leak of p->refer.

chan_sip: Fix theoretical leak of p->refer.

If transmit_refer is called when p->refer is already allocated,
it leaks the previous allocation.  Updated code to always free
previous allocation during a new allocation.  Also instead of
checking if we have a previous allocation, always create a
clean record.

ASTERISK-15242 #close
Reported by: David Woolley
Review: https://reviewboard.asterisk.org/r/4160/
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Merged revisions 428117 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 428118 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@428119 65c4cc65-6c06-0410-ace0-fbb531ad65f3