Fix Record-Route parsing for large headers.

Record-Route parsing copied the header into a char[256] array, which can
be a problem if the header is longer than that. This patch parses the
header in place, without the copy, avoiding the issue.

In addition to the original patch, I added a unit test for the new
get_in_brackets_const function.

(closes issue ASTERISK-20837)
Reported by: Corey Farrell
Patches:
	chan_sip-build_route-optimized-rev1.patch uploaded by Corey Farrell (license 5909)
	(with minor changes by dlee)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@379392 f38db490-d61c-443f-a65b-d21fe96a405b