Fix a panic bug on key event on Windows.

[NaokiM03]

Hi.

Fixed following issue. https://github.com/enigo-rs/enigo/issues/121

The policy of the fix was to follow the README of winapi-rs.

How do I create an instance of a union?

Use std::mem::zeroed() to create an instance of the union, and then assign the value you want using one of the variant methods.