scanner: fix error reporting for last comments (#591)

When an invalid TOML expression ends with a comment before the end of
file, the decode error would take a nil from scanComment, which is not
part of the document.

Fixes #588

unmarshaler_test.go

@@ -1740,6 +1740,12 @@ func TestIssue586(t *testing.T) {
 	require.Error(t, err)
 }
 
+func TestIssue588(t *testing.T) {
+	var v interface{}
+	err := toml.Unmarshal([]byte(`a=[1#`), &v)
+	require.Error(t, err)
+}
+
 //nolint:funlen
 func TestUnmarshalDecodeErrors(t *testing.T) {
 	examples := []struct {