Fix panics in cio/io_unix.go

Add nil checks before closing fifos to avoid panic when fifo was never
opened.

Signed-off-by: Cody Roseborough <crrosebo@amazon.com>

cio/io_unix_test.go

@@ -0,0 +1,41 @@
+// +build !windows
+
+package cio
+
+import (
+	"context"
+	"path/filepath"
+	"testing"
+
+	"github.com/gotestyourself/gotestyourself/assert"
+)
+
+func TestOpenFifos(t *testing.T) {
+	scenarios := []*FIFOSet{
+		{
+			Config: Config{
+				Stdin:  "",
+				Stdout: filepath.Join("This/does/not/exist", "test-stdout"),
+				Stderr: filepath.Join("This/does/not/exist", "test-stderr"),
+			},
+		},
+		{
+			Config: Config{
+				Stdin:  filepath.Join("This/does/not/exist", "test-stdin"),
+				Stdout: "",
+				Stderr: filepath.Join("This/does/not/exist", "test-stderr"),
+			},
+		},
+		{
+			Config: Config{
+				Stdin:  "",
+				Stdout: "",
+				Stderr: filepath.Join("This/does/not/exist", "test-stderr"),
+			},
+		},
+	}
+	for _, scenario := range scenarios {
+		_, err := openFifos(context.Background(), scenario)
+		assert.Assert(t, err != nil, scenario)
+	}
+}