With aa3f5023e3, pScreen->devPrivate now is
initialized only once, which uncovered a silent bug in xnestOpenScreen:
It's NULL'ing the pScreen->devPrivate pointer which already had been
initialized by previous miScreenDevPrivateInit() call.
Fixes: aa3f5023e3
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1495>