fix(NODE-2129): fix sporadic AcquireCredentialsHandle error

[rose-m]

Fixes a sporadic error that occurred when calling the AcquireCredentialsHandleW SSPI function. This was caused by uninitialized struct members. The authIdentity variable was allocated on the stack but not initialized. Depending on the stack's state, that caused the authIdentity.DomainLength or potentially the authIdentity.Domain members to have unpredictable content. Ensuring proper nulled initialization fixes this.

This PR also includes a fix for a memory leak in the to_wstring function. The allocated output array was never deleteed.

We tested this fix as part of mongosh in a Windows environment with Kerberos.

@addaleax 🙇

[addaleax]

👍

[dariakp]

I defer to people more well-versed in C to review this code

[nbbeeken]

LGTM