egl: Fix _eglPointerIsDereferencable w/o mincore()

egl: Fix _eglPointerIsDereferencable w/o mincore()

On platforms without mincore(), _eglPointerIsDereferencable()
currently just checks whether p != NULL. This is not sufficient:
In the Wayland platform code (i.e., in get_wl_surface_proxy()),
_eglPointerIsDereferencable() is called on the version field
of `struct wl_egl_window` which is 3 on current versions of
Wayland. This causes a segfault when trying to dereference p.

Fix this behavior by assuming that the first page of the
process is never dereferencable.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3103>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3103>