Add a null pointer check to CString::new

This also removes checks in other methods of `CString`

Breaking changes:
* `CString::new` now fails if `buf` is null. To avoid this add a check
before creatng a new `CString` .
* The `is_null` and `is_not_null` methods are deprecated, because a
`CString` cannot be null.
* Other methods which used to fail if the `CString` was null do not fail anymore

[breaking-change]