Wrong side checked when handling cyclical detection.

In `visited.add`, `lhs` was checked for an empty map/slice when handling
the `rhs` side of the cyclical detection.

Author: yazgazan

diff/diff.go

@@ -190,7 +190,7 @@ func (v *visited) add(lhs, rhs reflect.Value) error {
 		}
 		v.lhs = append(v.lhs, lhs.Pointer())
 	}
-	if canAddr(rhs) && !isEmptyMapOrSlice(lhs) {
+	if canAddr(rhs) && !isEmptyMapOrSlice(rhs) {
 		if inPointers(v.rhs, rhs) {
 			return ErrCyclic
 		}

diff/diff_test.go

@@ -616,6 +616,9 @@ func TestCircular(t *testing.T) {
 				0: "foo",
 			},
 		},
+		1: []interface{}{
+			"bar", "baz",
+		},
 	}
 	emptySlice := map[int]interface{}{
 		0: []interface{}{},
@@ -644,6 +647,8 @@ func TestCircular(t *testing.T) {
 		{lhs: first, rhs: second, wantError: true},
 		{lhs: first, rhs: notCyclic, wantError: true},
 		{lhs: notCyclic, rhs: first, wantError: true},
+		{lhs: notCyclic, rhs: emptySlice, wantError: false},
+		{lhs: notCyclic, rhs: emptyMap, wantError: false},
 		{lhs: notCyclic, rhs: notCyclic},
 		{lhs: emptySlice, rhs: emptySliceNotRepeating},
 		{lhs: emptySliceNotRepeating, rhs: emptySlice},