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

scanner.go

@@ -118,7 +118,7 @@ func scanComment(b []byte) ([]byte, []byte) {
 		}
 	}
 
-	return b, nil
+	return b, b[len(b):]
 }
 
 func scanBasicString(b []byte) ([]byte, []byte, error) {

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 {