print IP{} as "zero IP" instead of "invalid IP"

It's a bit more precise.

Signed-off-by: Josh Bleecher Snyder <[email protected]>

Author: josharian

example_test.go

@@ -45,7 +45,7 @@ func ExampleIP_properties() {
 	w.Flush()
 	// Output:
 	// String()           Zone()  IsZero()  Is4()  Is6()  Is4in6()
-	// invalid IP                 true      false  false  false
+	// zero IP                    true      false  false  false
 	// 192.0.2.3                  false     true   false  false
 	// 2001:db8::68               false     false  true   false
 	// 2001:db8::68%eth0  eth0    false     false  true   false

netaddr.go

@@ -821,7 +821,7 @@ func (ip IP) Prior() IP {
 func (ip IP) String() string {
 	switch ip.z {
 	case z0:
-		return "invalid IP"
+		return "zero IP"
 	case z4:
 		return ip.string4()
 	default:

netaddr_test.go

@@ -861,7 +861,7 @@ func TestLessCompare(t *testing.T) {
 	}
 	sort.Slice(values, func(i, j int) bool { return values[i].Less(values[j]) })
 	got := fmt.Sprintf("%s", values)
-	want := `[invalid IP 1.2.3.4 8.8.8.8 ::1 ::1%foo ::2]`
+	want := `[zero IP 1.2.3.4 8.8.8.8 ::1 ::1%foo ::2]`
 	if got != want {
 		t.Errorf("unexpected sort\n got: %s\nwant: %s\n", got, want)
 	}
@@ -874,7 +874,7 @@ func TestIPStringExpanded(t *testing.T) {
 	}{
 		{
 			ip: IP{},
-			s:  "invalid IP",
+			s:  "zero IP",
 		},
 		{
 			ip: mustIP("192.0.2.1"),