Ensure websocket PING/PONG token has length 8 (RIPD-1670)

src/ripple/server/impl/BaseWSPeer.h

@@ -169,6 +169,7 @@ BaseWSPeer(
         io_service, journal)
     , request_(std::move(request))
     , timer_(io_service)
+    , payload_ ("12345678") // ensures size is 8 bytes
 {
 }