drawable.surface: Return nil if there is a surface

Before this commit, it would return a NULL pointer as a lightuserdata.

Signed-off-by: Uli Schlachter <psychon@znc.in>

objects/drawable.c

@@ -124,8 +124,11 @@ drawable_set_geometry(lua_State *L, int didx, area_t geom)
 static int
 luaA_drawable_get_surface(lua_State *L, drawable_t *drawable)
 {
+    if (drawable->surface)
         /* Lua gets its own reference which it will have to destroy */
         lua_pushlightuserdata(L, cairo_surface_reference(drawable->surface));
+    else
+        lua_pushnil(L);
     return 1;
 }