Replace fe->preset_menu when we change midend.

Replace fe->preset_menu when we change midend.

Thanks to Rocco Matano for reporting that in the -DCOMBINED version of
the Windows front end, switching games causes a crash because the
presets menu from the old midend is still left over in fe, and its
presence inhibits the setup code from making a new one. Now we throw
it out at the same time as we throw out the old midend itself.

Also, the condition 'if (!fe->preset_menu)' was misguided. I think the
point of that was to avoid pointlessly tearing down and rebuilding the
preset menu when we're _not_ changing game - but that's a cost too
small to worry about if it causes the slightest trouble. Now
fe->preset_menu should always be NULL at that point in the function,
so I've replaced the if with an assert.