When anchoring an annotation with a position and a quote selector, if
the position selector fails, then PDF anchoring searches page contents
starting with those pages nearest the position.

If the position selector's `start` offset was greater than the length of
the PDF's text, `prioritizePages` would try to fetch the text of page
indexes beyond the valid range, causing PDF.js to throw an exception and
quote anchoring to fail.

Fix this by adding a missing bounds check.

This is a partial fix for #558. It fixes anchoring of one of two test
annotations on that page. The other fails due to differences in the
extracted text between the HTML and PDF versions of the article.