Using dynamic address causes SEGV

[alanconway]

Test code

func TestSendReceive(t *testing.T) {
	c, err := amqp.Dial("amqp://")
	require.NoError(t, err)
	defer c.Close()
	ssn, err := c.NewSession()
	require.NoError(t, err)
	r, err := ssn.NewReceiver(amqp.LinkAddressDynamic())
	require.NoError(t, err)
	s, err := ssn.NewSender(amqp.LinkAddress(r.Address()))
	require.NoError(t, s.Send(context.Background(), amqp.NewMessage([]byte("hello"))))
	m, err := r.Receive(context.Background())
	require.NoError(t, err)
	assert.Equal(t, "hello", string(m.GetData()))
}

Result:

--- FAIL: TestSendReceive (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x6b71dd]

goroutine 248 [running]:
testing.tRunner.func1(0xc0002cc000)
	/usr/lib/golang/src/testing/testing.go:874 +0x3a3
panic(0x7dfca0, 0xb64460)
	/usr/lib/golang/src/runtime/panic.go:679 +0x1b2
pack.ag/amqp.attachLink(0xc000290180, 0xc0002a8040, 0xc0000edf58, 0x1, 0x1, 0x0, 0x0, 0x0)
	/home/aconway/go/src/pack.ag/amqp/client.go:981 +0x54d
pack.ag/amqp.(*Session).NewReceiver(0xc000290180, 0xc0000edf58, 0x1, 0x1, 0x0, 0x0, 0x0)
	/home/aconway/go/src/pack.ag/amqp/client.go:311 +0x81
pack.ag/amqp_test.TestSendReceive(0xc0002cc000)
	/home/aconway/go/src/pack.ag/amqp/local_test.go:48 +0x1a0
testing.tRunner(0xc0002cc000, 0x86d640)
	/usr/lib/golang/src/testing/testing.go:909 +0xc9
created by testing.(*T).Run
	/usr/lib/golang/src/testing/testing.go:960 +0x350
exit status 2
FAIL	pack.ag/amqp	0.032s