va: reject NULL display in vaGetDisplayWl()

Add a null-pointer check for the display parameter at the entry of vaGetDisplayWl(), returning NULL
immediately instead of proceeding to backend initialization which would crash with a null
dereference.

This aligns the Wayland constructor with the X11 and DRM constructors which already validate their
native display/fd arguments early.

Signed-off-by: Carl.Zhang <carl.zhang@intel.com>

va/wayland/va_wayland.c

diff --git a/va/wayland/va_wayland.c b/va/wayland/va_wayland.c
index 759b055..2f4c42c 100644
--- a/va/wayland/va_wayland.c
+++ b/va/wayland/va_wayland.c
@@ -116,6 +116,9 @@
     struct VADriverVTableWayland *vtable;
     unsigned int i;
 
+    if (!display)
+        return NULL;
+
     pDisplayContext = va_newDisplayContext();
     if (!pDisplayContext)
         return NULL;