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