[platform-win] Add null check to pOldDACL

[xmkg]

The new_ACL function assumes that pOldDACL ptr would be non-empty after a successful GetNamedSecurityInfo call, but that premise does not exist

Excerpt from GetNamedSecurityInfo docs:

A pointer to a variable that receives a pointer to the DACL in the
returned security descriptor or NULL if the security descriptor has
no DACL. The returned pointer is valid only if you set the DACL_SECURITY_INFORMATION
flag. Also, this parameter can be NULL if you do not need the DACL.

NULL DACL is a valid return value, indicating that the queried security info has no DACL (i.e., no access restrictions).

The patch adds a null check to fix that.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.65%. Comparing base (dd0b47c) to head (ef1e7c5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4735   +/-   ##
=======================================
  Coverage   87.65%   87.65%           
=======================================
  Files         258      258           
  Lines       14124    14124           
=======================================
  Hits        12379    12379           
  Misses       1745     1745           

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