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: a4210fe0 ("xfree86: Change displays array to pointers array to fix invalid pointer issues after table reallocation")
Signed-off-by: Zoltán Böszörményi zboszor@gmail.com