Get out of the infinite loop by allowing to scan backwards when the current token is a comment.

This fixes issue #9002.
@peterflynn `do { } while (!(ctx.pos.line === 0 && ctx.pos.ch === 0))` logic is not the same as the current logic which stops as soon as hitting a `{` or `}` for a normal css file, or a `}` in the top level for a preprocessor file. So I'll use your prior suggested code.