Work around root directory Open/Stat corner case leading to panic

See https://github.com/spf13/afero/pull/146#issuecomment-470840725

zipfs/file.go

@@ -148,7 +148,12 @@ func (f *File) Readdirnames(count int) (names []string, err error) {
 	return
 }
 
-func (f *File) Stat() (os.FileInfo, error) { return f.zipfile.FileInfo(), nil }
+func (f *File) Stat() (os.FileInfo, error) {
+	if f.zipfile == nil {
+		return &pseudoRoot{}, nil
+	}
+	return f.zipfile.FileInfo(), nil
+}
 
 func (f *File) Sync() error { return nil }
 

zipfs/zipfs_test.go

@@ -38,6 +38,17 @@ func TestZipFS(t *testing.T) {
 		t.Errorf("expected to get <aaaabbbb>, got <%s>", string(buf))
 	}
 
+	d, err := a.Open("/")
+	if d == nil {
+		t.Error(`Open("/") returns nil`)
+	}
+	if err != nil {
+		t.Errorf(`Open("/"): err = %v`, err)
+	}
+	if s, _ := d.Stat(); !s.IsDir() {
+		t.Error(`expected root ("/") to be a directory`)
+	}
+
 	buf = make([]byte, 8192)
 	if n, err := f.Read(buf); err != nil {
 		t.Error(err)
@@ -51,6 +62,7 @@ func TestZipFS(t *testing.T) {
 		path string
 		dir  bool
 	}{
+		{"/", true},
 		{"testDir1", true},
 		{"testDir1/testFile", false},
 		{"testFile", false},