When relaying messages in multiplayer mode.
Could cause a crash in case a malicious client sends a bogus packet and
for those cases where a peer has just disconnected and a message arrive
from another peer with the disconnected one as destination.
(cherry picked from commit 17be67b8c7)
The WebSocketMultiplayerPeer was relaying the same message two times,
both in _server_relay and _process_multiplayer (which was only supposed
to store the packet, given the server was one of the destination).
_process_multiplayer now only store the packet, and calls _server_relay
which will relay the message to other clients if needed.
(cherry picked from commit d1539db2c6)
