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 Trac #15163.
It's easy to fix.
NB: this does not completely fix Trac Trac #15163; more to come