libepoxy: add patch to fix segfaults when using in X11 without GLX

This adds a patch cherry-picked from libepoxy 1.4.2, which adds missing
NULL-pointer checks and avoids segmentation faults when using libepoxy
under X11, when the server does not have the GLX extension, or it is
disabled -- and applications can still use EGL.

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>