dri_util: fail driCreateNewScreen if InitScreen is NULL

Without this, X doesn't start with UMS on r300g.

NOTE: This is a candidate for the 7.9 and 7.10 branches.

Signed-off-by: Paulo Zanoni <pzanoni@mandriva.com>
Signed-off-by: Brian Paul <brianp@vmware.com>

src/mesa/drivers/dri/common/dri_util.c

@@ -790,6 +790,9 @@ driCreateNewScreen(int scrn,
     static const __DRIextension *emptyExtensionList[] = { NULL };
     __DRIscreen *psp;
 
+    if (driDriverAPI.InitScreen == NULL)
+	return NULL;
+
     psp = calloc(1, sizeof *psp);
     if (!psp)
 	return NULL;