Add missing check to isReflCoVar_maybe

isReflCoVar_maybe is called, by CoreLint, on all sorts of
Vars (tyvars, term vars, coercion vars).  But it was silently
assuming that it was always called on a CoVar, and as a result
could crash fatally.  This is the immediate cause of the panic
in Trac #15163.

It's easy to fix.

NB: this does not completely fix Trac #15163; more to come