fix unsound APIs that could lead to UB

Affected versions of this crate API could use uninitialized memory with some APIs in special
cases, like use the API in none generator context. This could lead to UB.
The flaw was corrected by <https://github.com/Xudong-Huang/generator-rs/issues/9&gt;
<https://github.com/Xudong-Huang/generator-rs/issues/11&gt;
<https://github.com/Xudong-Huang/generator-rs/issues/13&gt;
<https://github.com/Xudong-Huang/generator-rs/issues/14&gt;
This patch fixes all those issues above.