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>
5 jobs for fix-crash-if-no-virtual in 6 minutes and 17 seconds (queued for 8 seconds)