GetSystemWindowsDirectoryW doesn't like nil buffers

Author: grynspan

Sources/Testing/Support/FileHandle.swift

@@ -732,7 +732,9 @@ let rootDirectoryPath: String = {
   // contains the system Windows directory. For an explanation of the difference
   // between the Windows directory and the _system_ Windows directory, see
   // https://devblogs.microsoft.com/oldnewthing/20140723-00/?p=423 .
-  let count = GetSystemWindowsDirectoryW(nil, 0)
+  let count = withUnsafeTemporaryAllocation(of: wchar_t.self, capacity: 1) { buffer in
+    GetSystemWindowsDirectoryW(buffer.baseAddress!, buffer.count)
+  }
   if count > 0 {
     withUnsafeTemporaryAllocation(of: wchar_t.self, capacity: Int(count) + 1) { buffer in
       _ = GetSystemWindowsDirectoryW(buffer.baseAddress!, UINT(buffer.count))