fixes reload Chan_dahdi memory leak caused by variables

fixes reload Chan_dahdi memory leak caused by variables

chan_dahdi reloading with variables set via setvar in chan_dahdi.conf would
stay in the dahdi_pvt structs for individual channels (causing them to just
continue adding the new ones to the list) and also there was a memory leak
causes by the conf objects. This patch resolves both of these by using
ast_variables_destroy during the loading process.

(closes issue #17450)
Reported by: nahuelgreco
Patches:
	patch.diff uploaded by jrose (license 1225)
	Tested by: tilghman, jrose

Review: https://reviewboard.asterisk.org/r/1170/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.6.2@313432 65c4cc65-6c06-0410-ace0-fbb531ad65f3