When taking a substring and a negative length is provided, instead of just

ignoring it, allow this to mean that we want that many characters off of
the end of the string so that ${EXTEN:0:$[${LEN(${EXTEN}) - 1]} can become
${EXTEN:0:-1}.  (issue #7586, Corydon)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@39659 65c4cc65-6c06-0410-ace0-fbb531ad65f3