Enable loading of PDFs with undefined or missing stream lengths

After #3376 landed, there is support for loading of PDFs with incorrect stream lengths (commit https://github.com/yurydelendik/pdf.js/commit/4d9ee7b53002cc34e42b503db31530deea50c62c). This code works equally well when a stream length is undefined, so this PR enables loading of PDFs with undefined stream lengths.

Fixes #1293.

/cc @yurydelendik