rpcserver+itest: skip err from FlapCount

We need to make sure the `ListPeers` to be robust against errors from
the `FlapCount` so this RPC won't fail due to no flap count info. Also
updated the itest to check this field.

itest/lnd_misc_test.go

@@ -257,6 +257,17 @@ func testListChannels(ht *lntest.HarnessTest) {
 	assertChannelConstraintsEqual(
 		ht, aliceChannel.RemoteConstraints, bobChannel.LocalConstraints,
 	)
+
+	// Finally we assert that the flap count is updated as expected.
+	resp := alice.RPC.ListPeers()
+
+	// Assert Alice only have one peer.
+	require.Len(ht, resp.Peers, 1)
+	for _, p := range resp.Peers {
+		// The channel open event resulted in an online event, so we
+		// expect the flap count to be 1.
+		require.EqualValues(ht, 1, p.FlapCount)
+	}
 }
 
 // testMaxPendingChannels checks that error is returned from remote peer if