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

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@249562 138bc75d-0d04-0410-961f-82ee72b054a4