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.

Change-Id: I94e6a52c8832ab34aa1c7653d0ab9fce6126bf8a
Reviewed-on: https://go-review.googlesource.com/46450
Reviewed-by: Than McIntosh <thanm@google.com>
2 files changed