env::vars panics if environ is null

[jorickert]

The following code panics:

unsafe { libc::clearenv(); };
assert_eq!(env::vars().count(), 0);

The problem is that env assumes that environ is not null. See Rust env implementation

The clearenv(3 ) function on linux is often implemented by freeing environ and setting it to null. See uclibc and glibc

In my opinion the stdlib could handle this more gracefully and return an empty iterator instead of panicking

[BurntPizza]

I'm working on a PR.