Fix an error introduced by copying and pasting the handling of the >= operator

Fix an error introduced by copying and pasting the handling of the >= operator
for the MATH function.  If a single equal sign was used as an operator, the
function would treat it is as if it were the >= operator.  Now, it properly
handles it as an invalid operator.
(issue #8665, patch by tempest1)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.2@48955 65c4cc65-6c06-0410-ace0-fbb531ad65f3