Potential fix for code scanning alert no. 243: Setting a DACL to NULL in a SECURITY_DESCRIPTOR

[JaviLendi]

Potential fix for https://github.com/JaviLendi/PrettySumatraPDF/security/code-scanning/243

Use a non-NULL ACL instead of a NULL DACL. The safest minimal-change fix is to build an empty DACL (an allocated ACL with no ACEs) and set that on the security descriptor. This keeps the object protected (no one gets access by default) and removes the dangerous NULL DACL pattern flagged by CodeQL.

In src/utils/WinUtil.cpp, inside ResetRegKeyAcl (around lines 567–577), replace the SetSecurityDescriptorDacl(..., nullptr, ...) block with:

1. Allocate/initialize a small ACL buffer (ACL dacl; InitializeAcl(...)).
2. Call SetSecurityDescriptorDacl(&secdesc, TRUE, &dacl, FALSE).
3. Check return values and early-return safely on failure.
4. Keep RegSetKeySecurity call only after successful descriptor setup.

No new imports/dependencies are required; all used WinAPI symbols are already available from existing Windows headers in this file.

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