Fix a hang issue due to inconsistent page number in the test file

Pdfium reads the page number from the field of '/Count' but it can't
load the number assigned by this field due to the damaged data. Add a
check to ensure that the required page should be one of loaded pages.

BUG=406090
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/477873003