res_pjsip_refer: Ensure Refer-To is NULL terminated and parse it as a URI.

There is no guarantee that when we get a Refer-To that it will be NULL terminated.
As the URI parsing function requires it to be we now NULL terminate it.

Additionally parsing the Refer-To as a 'To' header is needless and it can
simply be done as a URI. This also fixes a problem where certain Refer-To headers
would not be parsed as a 'To' header causing the REFER to fail.

ASTERISK-24508 #close
Reported by: Beppo Mazzucato

Review: https://reviewboard.asterisk.org/r/4187/
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Merged revisions 428195 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@428196 f38db490-d61c-443f-a65b-d21fe96a405b