util: check RNG error before slicing url-safe random string

A crypto/rand failure sliced empty output and panicked.

hscontrol/util/string_random_test.go

@@ -0,0 +1,24 @@
+package util
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestEncodeRandomURLSafeChecksErrorFirst ensures the URL-safe random string
+// encoder returns ("", err) on an RNG failure instead of slicing the empty
+// base64 of nil bytes and panicking.
+func TestEncodeRandomURLSafeChecksErrorFirst(t *testing.T) {
+	require.NotPanics(t, func() {
+		s, err := encodeRandomURLSafe(nil, 32, assert.AnError)
+		assert.Empty(t, s)
+		require.Error(t, err)
+	})
+
+	s, err := encodeRandomURLSafe(bytes.Repeat([]byte{0x1}, 32), 32, nil)
+	require.NoError(t, err)
+	assert.Len(t, s, 32)
+}