runtime: scan write barrier buffer conservatively

runtime: scan write barrier buffer conservatively

    In gccgo, we insert the write barriers in the frontend, and so we
    cannot completely prevent write barriers on stack writes. So it
    is possible for a bad pointer appearing in the write barrier
    buffer. When flushing the write barrier, treat it the same as
    sacnning the stack. In particular, don't mark a pointer if it
    does not point to an allocated object. We already have similar
    logic in greyobject. With this, hopefully, we can prevent an
    unallocated object from being marked completely.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/190599

From-SVN: r274598