FoundationEssentials: correct directory enumeration on Windows

When enumerating a directory on Windows, we would fail to increment the
iterator as we would increment _after_ the return. Insert a `defer`'ed
increment on the iterator to allow us to track the state properly.
Because the iteration was may have completed on the previous run, we
need to track an additional bit of state as `bValid` which provides us
an indication if iteration has completed.

Author: compnerd

Sources/FoundationEssentials/FileManager/FileOperations+Enumeration.swift

@@ -23,6 +23,7 @@ struct _Win32DirectoryContentsSequence: Sequence {
         }
 
         private var hFind: HANDLE?
+        private var bValid: Bool = true
         private var ffdData: WIN32_FIND_DATAW = .init()
         private var prefix: String = ""
         private var slash: Bool
@@ -69,6 +70,7 @@ struct _Win32DirectoryContentsSequence: Sequence {
 
         func next() -> Element? {
             guard let hFind else { return nil }
+            guard bValid else { return nil }
             repeat {
                 let name = withUnsafeBytes(of: ffdData.cFileName) {
                     String(decodingCString: $0.baseAddress!.assumingMemoryBound(to: WCHAR.self), as: UTF16.self)
@@ -80,8 +82,10 @@ struct _Win32DirectoryContentsSequence: Sequence {
 
                 let prefixed: String =
                     prefix + name + ((slash && ffdData.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY == FILE_ATTRIBUTE_DIRECTORY) ? #"\"# : "")
+                defer { bValid = FindNextFileW(hFind, &ffdData) }
                 return Element(fileName: name, fileNameWithPrefix: prefixed, dwFileAttributes: ffdData.dwFileAttributes)
             } while FindNextFileW(hFind, &ffdData)
+            bValid = false
             return nil
         }
     }