Fix Record-Route parsing for large headers.

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)
........

Merged revisions 379392 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 379393 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@379394 65c4cc65-6c06-0410-ace0-fbb531ad65f3