FS#2901: Fix lessphp on PHP 5.2.0

Upstream issue for this change: leafo/lessphp#522

For me this fixes the problem on PHP 5.2.0 on Debian Etch. This change might break the less compiler (on all PHP versions). I haven't tried understanding what exactly the code does and if this change matters or not. Earlier in the code the same two objects are compared using ``===`` so this change might be okay or it might also be intentional that in this case it is not ``!==`` but ``!=``.