This is interesting. There is no libva-dbg in Debian, so I decided to compile libva myself. I got the source from http://ftp.de.debian.org/debian/pool....8.orig.tar.gz
to find out that my own libva doesn't crash at all.
So it's either something broken in Debian's patches (although I failed to find anything that would strike me as apparently wrong in their patchset) or I don't know. However I can confirm that X11 libraries are receiving a NULL pointer.