This is the ebuild:
https://gitweb.gentoo.org/repo/gentoo.git/tree/media-libs/mesa/mesa-17.2.2.ebuild

Patches would be listed in a PATCHES variable or applied in src_prepare().  The
ebuild has only eapply_user there, which would apply patches in
/etc/portage/patches/media-libs/mesa (or a version specific directory), which I
don't have.  Hence I believe there are no patches applied on my system.

I will extract the sources again and will try to attach GDB, while DrKonqi
(KDE's bug-reporting tool) has the process stopped, the next time I reproduce
the crash in the coming days.

I will also install using the 9999 / git ebuild and see whether I can reproduce
there.