Correct measurement for owner drawn menu items with only unchecked bitmap.

Correct measurement for owner drawn menu items with only unchecked bitmap.

Check for either checked or unchecked bitmap being specified for the item
instead of checking the checked/normal bitmap twice which was an obvious typo
and also resulted in items with unchecked bitmap only not being measured
correctly (although I'm not sure if this is actually a supported use case).

Closes #12846.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66602 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775