Gallery Permalinks using /%category%/%postname%/ return 404

[MtDewVirus]

When using /%category%/%postname%/ as the Custom Structure for Common Settings, the image Gallery permalinks return a 404.

Default, Day and Name, Month and Name, and Numeric all work. Also tested using /%year%/%postname%/ which worked.

Tested on r7399

Duplicate caused by the same cause of #4699