chan_sip: Fix buffer overrun in sip_sipredirect.

sip_sipredirect uses sscanf to copy up to 256 characters to a stacked buffer
of 256 characters.  This patch reduces the copy to 255 characters to leave
room for the string null terminator.

ASTERISK-25722 #close

Change-Id: Id6c3a629a609e94153287512c59aa1923e8a03ab