Allow Python overrides for materials.

source/blender/blenkernel/intern/material.c

@@ -806,9 +806,13 @@ void assign_material_id(ID *id, Material *ma, short act)
 	if (act > MAXMAT) return;
 	if (act < 1) act = 1;
 
+	/* this is needed for Python overrides,
+	 * we just have to take care that the UI can't do this */
+#if 0
 	/* prevent crashing when using accidentally */
 	BLI_assert(id->lib == NULL);
 	if (id->lib) return;
+#endif
 
 	/* test arraylens */