xnest: fix segfault in miCreateScreenResources()
With aa3f5023, 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: aa3f5023 Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net