drm/mgag200: fix null pointer dereference

we are referencing the pointer after doing alloc_apertures,
as alloc_apertures kzallocs, the kzalloc may fail and we get a NULL.

so we need to check for NULL before we dereference this pointer
Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/mgag200/mgag200_drv.c

@@ -47,6 +47,9 @@ static void mgag200_kick_out_firmware_fb(struct pci_dev *pdev)
 	bool primary = false;
 
 	ap = alloc_apertures(1);
+	if (!ap)
+		return;
+
 	ap->ranges[0].base = pci_resource_start(pdev, 0);
 	ap->ranges[0].size = pci_resource_len(pdev, 0);