Fix crash in parsepdf.c on filter&compression; fix another minor nit

The first commit "Warn for PDF's with compression + unsup'd filters" works around the crash described in issue #1721 by detecting the use of parameter definitions describing de-filtering algorithms to be used after decompression.  Since FontForge does not have any code to handle this additional filtering/defiltering, we detect the situation and **warn the user** of the unsupported PDF features and give up on the PDF file instead of crashing.  Thus this is a workaround rather than a fix.

The first commit closes #1721.

The second commit "startxref" is just something I noticed while trawling around parsepdf.c and it bothered me.  After fixing it one way (minimal changes), I realized that instead the code should be simplified by being made more *obvious* as to what was going on.