Fixed a few compilation warnings.

diff --git a/graphics/x11drv/bitmap.c b/graphics/x11drv/bitmap.c
index 67f3d08..484a7b6 100644
--- a/graphics/x11drv/bitmap.c
+++ b/graphics/x11drv/bitmap.c
@@ -585,8 +585,8 @@
  */
 Pixmap X11DRV_BITMAP_CreatePixmapFromBitmap( HBITMAP hBmp, HDC hdc )
 {
-    HGLOBAL hPackedDIB = NULL;
-    Pixmap pixmap = NULL;
+    HGLOBAL hPackedDIB = 0;
+    Pixmap pixmap = 0;
 
     /*
      * Create a packed DIB from the bitmap passed to us.