textbox: overwrite extents to 0 on nil

Signed-off-by: Julien Danjou <julien@danjou.info>

widgets/textbox.c

@@ -157,6 +157,8 @@ luaA_textbox_newindex(lua_State *L, awesome_token_t token)
                 d->extents = draw_text_extents(globalconf.connection, globalconf.default_screen,
                                                globalconf.font, d->text, d->len, &d->pdata).width;
             }
+            else
+                d->extents = 0;
         }
         break;
       case A_TK_WIDTH: