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:
- #34

go.mod

@@ -2,6 +2,7 @@ module github.com/ipfs/go-ipfs-files
 
 require (
 	github.com/crackcomm/go-gitignore v0.0.0-20170627025303-887ab5e44cc3
+	github.com/stretchr/testify v1.7.0 // indirect
 	golang.org/x/sys v0.0.0-20190302025703-b6889370fb10
 )
 

go.sum

@@ -1,4 +1,15 @@
 github.com/crackcomm/go-gitignore v0.0.0-20170627025303-887ab5e44cc3 h1:HVTnpeuvF6Owjd5mniCL8DEXo7uYXdQEmOP4FJbV5tg=
 github.com/crackcomm/go-gitignore v0.0.0-20170627025303-887ab5e44cc3/go.mod h1:p1d6YEZWvFzEh4KLyvBcVSnrfNDDvK2zfK/4x2v/4pE=
+github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/testify v1.7.0 h1:nwc3DEeHmmLAfoZucVR881uASk0Mfjw8xYJ99tb5CcY=
+github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 golang.org/x/sys v0.0.0-20190302025703-b6889370fb10 h1:xQJI9OEiErEQ++DoXOHqEpzsGMrAv2Q2jyCpi7DmfpQ=
 golang.org/x/sys v0.0.0-20190302025703-b6889370fb10/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

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
 	}