derp: fix meshing accounting edge case bug

If a peer is connected to multiple nodes in a region (so
multiForwarder is in use) and then a node restarts and re-sends all
its additions, this bug about whether an element is in the
multiForwarder could cause a one-time flip in the which peer node we
forward to.  Note a huge deal, but not written as intended.

Thanks to @lewgun for the bug report in #2141.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derp_server.go

@@ -1275,7 +1275,7 @@ func (s *Server) AddPacketForwarder(dst key.Public, fwd PacketForwarder) {
 			return
 		}
 		if m, ok := prev.(multiForwarder); ok {
-			if _, ok := m[fwd]; !ok {
+			if _, ok := m[fwd]; ok {
 				// Duplicate registration of same forwarder in set; ignore.
 				return
 			}

derp/derp_test.go

@@ -712,6 +712,7 @@ func TestForwarderRegistration(t *testing.T) {
 	// Adding a dup for a user.
 	wantCounter(&s.multiForwarderCreated, 0)
 	s.AddPacketForwarder(u1, testFwd(100))
+	s.AddPacketForwarder(u1, testFwd(100)) // dup to trigger dup path
 	want(map[key.Public]PacketForwarder{
 		u1: multiForwarder{
 			testFwd(1):   1,