remove misplaced AVER

(defstruct nil x) is clearly never going to work, but doesn't
deserve the scary "probably a bug in SBCL itself" message
more targetted warnings for probable DEFSTRUCT syntax errors

Some keyword defstruct slot names are more deserving of warnings
than others.  In particular, slot names matching defstruct
options (from name-and-options) are highly suspicious, and likely
never to be intended, as there is essentially always a different
way of writing what is meant.

The exception is when the CONC-NAME defstruct option is null,
which makes defstruct slots effectively have symbolic identity
rather than the normal stringish nature; don't emit full
warnings under those circumstances, even though doing
  (defstruct (foo :conc-name) (:constructor))
is a pretty spectacularly bad idea.