A couple of small improvements in the "SaveDocument" handler in `src/core/worker.js`
A couple of small improvements in the "SaveDocument" handler in `src/core/worker.js`

 - Check that the "Info"-entry, in the XRef-trailer, is actually a dictionary before accessing it. This is similar to the `PDFDocument.documentInfo` method and follows the general principal of validating data carefully before accessing it, given how often PDF-software may create corrupt PDF files.

 - Slightly simplify the "XFA"-lookup, since there's no point in trying to fetch something from the empty dictionary.

Snuffleupagus/pdf.js,nawawi/pdf.js,nawawi/pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,Shoobx/pdf.js,macroplant/pdf.js,Snuffleupagus/pdf.js,showpad/mozilla-pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,timvandermeij/pdf.js,xavier114fch/pdf.js,macroplant/pdf.js,nawawi/pdf.js,caremerge/pdf.js,macroplant/pdf.js,caremerge/pdf.js,Snuffleupagus/pdf.js,Shoobx/pdf.js,xavier114fch/pdf.js,Shoobx/pdf.js