msgpack-rpc: Close channel on invalid msgpack RPC

- When an invalid msgpack RPC msg is received from a channel
  we now close that channel all calls on that channel fail with
  an error message.

src/nvim/msgpack_rpc/channel.c

@@ -436,6 +436,11 @@ static void handle_request(Channel *channel, msgpack_object *request)
                                      &error,
                                      NIL,
                                      &out_buffer));
+    char buf[256];
+    snprintf(buf, sizeof(buf),
+             "Channel %" PRIu64 " sent an invalid message, closing.",
+             channel->id);
+    call_set_error(channel, buf);
     return;
   }