Fix test failure on Windows caused by nil sys in mock FileInfo

Fix test failure caused by `panic` when checking if a file is hidden in
Windows. The hidden check uses `FileInfo.Sys()`. In tests the object is
a mock and that call returns `nil`. Regardless of mocking, we should
check for `nil` since according to docs that function can return `nil`.

Run `go mod tidy`.

Relates to:
- ipfs/go-ipfs-files#34


This commit was moved from ipfs/go-ipfs-files@8246860

files/is_hidden_windows.go

@@ -19,7 +19,11 @@ func isHidden(fi os.FileInfo) bool {
 		return true
 	}
 
-	wi, ok := fi.Sys().(*windows.Win32FileAttributeData)
+	sys := fi.Sys()
+	if sys == nil {
+		return false
+	}
+	wi, ok := sys.(*windows.Win32FileAttributeData)
 	if !ok {
 		return false
 	}