runtime: don't assume that _ = *s will panic if s is nil

runtime: don't assume that _ = *s will panic if s is nil

    With the gc toolchain apparently
            var s *string
            _ = *s
    is enough to panic with a nil pointer dereference. The gccgo compiler
    will simply discard the dereference, which I think is a reasonable and
    acceptable optimization. Change the tests to use an exported variable
    instead. The tests are not currently run, but they will be with a
    later patch to gotools.

    Reviewed-on: https://go-review.googlesource.com/46450

From-SVN: r249562