Once we hit AT_NULL, we need to bail out of the loop; not just the enclosing
switch. This fixes basic usage (e.g. cc --version) when AT_EXECPATH isn't
present on older branches (e.g. under qemu-user-static, at the moment),
where we would previously run off the end of ::environ.
Signed-off-by: Kyle Evans <kevans@FreeBSD.org>