Recursive mixin calls regression fix.

Fixes detection of recursive calls broken in https://github.com/less/less.js/commit/6d3414d217b881284e6a2b6fa0ce46446fdc0bbd. Fixes #1915 and #1928 (?).

@lukeapage Let me know if it's better to PR this to 2.0 branch instead (I'm not sure what state it is in for the moment).