TestServiceWithDefaultAddressPoolInit: avoid panic

Saw this in moby ci:

> 00:22:07.582 === RUN   TestServiceWithDefaultAddressPoolInit
> 00:22:08.887 --- FAIL: TestServiceWithDefaultAddressPoolInit (1.30s)
> 00:22:08.887 	daemon.go:290: [d905878b35bb9] waiting for daemon to start
> 00:22:08.887 	daemon.go:322: [d905878b35bb9] daemon started
> 00:22:08.888 panic: runtime error: index out of range [recovered]
> 00:22:08.889 	panic: runtime error: index out of range
> 00:22:08.889
> 00:22:08.889 goroutine 360 [running]:
> 00:22:08.889 testing.tRunner.func1(0xc42069d770)
> 00:22:08.889 	/usr/local/go/src/testing/testing.go:742 +0x29d
> 00:22:08.890 panic(0x85d680, 0xb615f0)
> 00:22:08.890 	/usr/local/go/src/runtime/panic.go:502 +0x229
> 00:22:08.890 github.com/docker/docker/integration/network.TestServiceWithDefaultAddressPoolInit(0xc42069d770)
> 00:22:08.891 	/go/src/github.com/docker/docker/integration/network/service_test.go:348 +0xb53
> .....

Apparently `out.IPAM.Config[0]` is not there, so to avoid panic, let's
check the size of `out.IPAM.Config` first.

Fixes: f7ad95cab9c

[v2: add logging of data returned by NetworkInspect()]
[v3: use assert.Assert to fail immediately]

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 69d3a8936b8d91e43290aadb9f7ac2cd2929dc61)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

integration/network/service_test.go

@@ -345,6 +345,8 @@ func TestServiceWithDefaultAddressPoolInit(t *testing.T) {
 
 	out, err := cli.NetworkInspect(context.Background(), name, types.NetworkInspectOptions{})
 	assert.NilError(t, err)
+	t.Logf("%s: NetworkInspect: %+v", t.Name(), out)
+	assert.Assert(t, len(out.IPAM.Config) > 0)
 	assert.Equal(t, out.IPAM.Config[0].Subnet, "20.20.0.0/24")
 
 	err = cli.ServiceRemove(context.Background(), serviceID)