1.0.18.27: fix DEFSTRUCT compilation when init-form type is vague

1.0.18.27: fix DEFSTRUCT compilation when init-form type is vague

 * Reported by Josh Morrison. Regression from the recent DEFSTRUCT
   constructor hacking.

   Slots initialized by constructor lambda-list arguments have their
   types checked, but those slots always initialized directly by the
   initform values did not -- wrap the DSD-DEFAULT in a THE.