compiler: avoid crashing on bad self-referential type

compiler: avoid crashing on bad self-referential type

    The compiler was crashing partway through emitting an error for a bad
    self-referential struct type (which refers to one of its own fields
    via an unsafe.Offset expression). Tweak the offset logic to check for
    this class of type error and exit cleanly.

    Fixes golang/go#24180

    Reviewed-on: https://go-review.googlesource.com/97855

From-SVN: r258112