Make domain.create().exit() safe

Calling `.exit()` on a domain that isn't in the domain stack should not clear the domain stack. It should do nothing.

Given:
```JavaScript
var domain = require('domain');

var a = domain.create();    a.name = 'a';
var b = domain.create();    b.name = 'b';
var c = domain.create();    c.name = 'c';

a.enter();    dumpDomains();
b.enter();    dumpDomains();
c.enter();    dumpDomains();

domain
  .create()
  .exit();    dumpDomains();

function dumpDomains() {
  console.log(domain._stack.map(function(d){ return d.name; }));
}
```

Expected output:
```
[ 'a' ]
[ 'a', 'b' ]
[ 'a', 'b', 'c' ]
[ 'a', 'b', 'c' ]
```

Actual output:
```
[ 'a' ]
[ 'a', 'b' ]
[ 'a', 'b', 'c' ]
[]
```

Fixes #6819