ast_exists_extension() does not return an ast_device_state, so change this

function to explicitly check for the int return value.  Also, make a few
other minor changes such as removing a variable.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@73886 65c4cc65-6c06-0410-ace0-fbb531ad65f3