res_pjsip_refer.c: Fix seg fault in process of Refer-to header.

The "Refer-to" header of an incoming REFER request is parsed by
pjsip_parse_uri().  That function requires the URI parameter to be NULL
terminated.  Unfortunately, the previous code added the NULL terminator by
overwriting memory that may not be safe.  The overwritten memory results
could be benign, memory corruption, or a segmentation fault.  Now the URI
is NULL terminated safely by copying the URI to a new chunk of memory with
the correct size to be NULL terminated.

ASTERISK-25814 #close

Change-Id: I32565496684a5a49c3278fce06474b8c94b37342