libnetwork: fix unhandled errors in tests (ineffassign)

    libnetwork/libnetwork_test.go:1014:8: ineffectual assignment to err (ineffassign)
        sbx1, err := controller.NewSandbox(containerID,
              ^
    libnetwork/libnetwork_test.go:1024:8: ineffectual assignment to err (ineffassign)
        sbx2, err := controller.NewSandbox("c2")
              ^

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libnetwork/libnetwork_test.go

@@ -1014,14 +1014,21 @@ func TestEndpointMultipleJoins(t *testing.T) {
 	sbx1, err := controller.NewSandbox(containerID,
 		libnetwork.OptionHostname("test"),
 		libnetwork.OptionDomainname("docker.io"),
-		libnetwork.OptionExtraHost("web", "192.168.0.1"))
+		libnetwork.OptionExtraHost("web", "192.168.0.1"),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
 	defer func() {
 		if err := sbx1.Delete(); err != nil {
 			t.Fatal(err)
 		}
 	}()
 
 	sbx2, err := controller.NewSandbox("c2")
+	if err != nil {
+		t.Fatal(err)
+	}
 	defer func() {
 		if err := sbx2.Delete(); err != nil {
 			t.Fatal(err)