xfree86: Fix NULL pointer dereference crash

screenp->displays[count] (passed to configDisplay() in
configScreen()) is NULL if there is no Virtual setting
in the configuration.

Fixes: f8a6be04 ("xfree86: Change
displays array to pointers array to fix invalid pointer issues
after table reallocation")

Signed-off-by: Zoltán Böszörményi <zboszor@gmail.com>
(cherry picked from commit ef89b664)
3 jobs for fix-crash-if-no-virtual-1.20 in 5 minutes and 9 seconds (queued for 10 seconds)