Fix a regression preventing page change in the Stepper in the debugger

This is a regression from https://github.com/mozilla/pdf.js/commit/ddd3c8fc2fe7b8715bed6d6f146c206d133b5dd6, which prevents changing pages in the Stepper.

(Apart from fixing the bug, for the sake of consistency, I also removed one instance of `parseInt`.)

@timvandermeij Can you please review this?