The setup code for CTRL_FILESELECT and CTRL_FONTSELECT is shared,
which means it's a mistake to test ctrl->fileselect.just_button in it
without first checking which control type we're actually dealing with.
UBsan picks this up by complaining that the just_button field contains
some byte value that's illegal for a boolean. I think it's also the
cause of an intermittent assertion failure reported recently, in which
dlg_fontsel_set finds that uc->entry is NULL when it never ought to
be. If the byte from the wrong union branch happened to be 0 by sheer
bad luck, that could give rise to exactly that failure.
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