menuconfig: Avoid crashing when leaving a menu not shown in parent

[leandrolanzieri]

This adds a catch to an exception that might occur when leaving a menu which is not shown by the parent menu. For an example of this refer to issue #93.

For the snippet given in #93 this ends up showing the top menu. I'm not sure if this is the expected behaviour (after all, the second definition of the choice is defined in the top menu), or 'Sub' menu should be shown. At least this seems to avoid the crash.