racket/class: add missing check for #<unsafe-undefined>

When a superclass method is called before the superclass
is initialized, then all field accesses need to be guarded.

Robby found this one.
racket/draw and racket/snip: avoid some undefined-checking wrappers

The changes to `pen%` and `brush%` are related to the
`racket/class` repair of the previous commit, while the
`style%` change is because I didn't look at it before.