[Windows] Fix calculating credential blob size

[hawkeye116477]

Earlier unecessary terminal sign was added to end of string.

[imnasnainaec]

According to Devin, this PR isn't a correct fix:

SIL.PasswordStore/Provider/WindowsProvider.cs:R28-34

Stored passwords can come back with extra garbage characters appended

The saved password is now recorded with a length that excludes its end marker (Encoding.Unicode.GetByteCount(password) at SIL.PasswordStore/Provider/WindowsProvider.cs:28-34), while the code that reads it back still relies on that marker to know where the password ends, so a retrieved password can contain trailing junk.
Impact: Users may get back a corrupted password (with random extra characters) instead of the one they saved.

Blob length excludes the terminating NUL while the reader scans for one

Marshal.StringToCoTaskMemUni allocates the string plus a terminating UTF-16 NUL, but only CredentialBlobSize bytes are persisted by CredWrite. With the new size (exact byte count of the characters, no terminator), the stored blob contains no NUL. On read, SIL.PasswordStore/Provider/WindowsProvider.cs:61 calls Marshal.PtrToStringUni(credential.CredentialBlob) with no length, so it keeps reading memory past the end of the returned blob until it happens to hit a zero word, producing extra characters or reading unrelated memory.

Fix: read using the known length, e.g. Marshal.PtrToStringUni(ptr, (int)(credential.CredentialBlobSize / 2)) (also handle odd sizes), or keep writing the terminator by adding sizeof(char) to the blob size.

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When asked if the existing code on master works, Devin says that it works for ASCII passwords but not for non-ASCII.

[hawkeye116477]

this PR isn't a correct fix
while the code that reads it back still relies on that marker to know where the password ends, so a retrieved password can contain trailing junk.

It's fine, just needs merging #7 also.

[imnasnainaec]

If the two PRs are so tightly coupled, please combine them into a single PR.