network: free packet on unknown wire id in read_from_packet

[janry]

    network_command_t::read_from_packet allocates an nwc by switch on
    p->get_id() and only reaches the receive(p) call (which transfers
    packet ownership to the nwc) when the switch produced one.  An
    unknown id fell through default, the function returned NULL with
    p still live, and the caller — socket_info_t::receive_nwc — set
    its own packet member to NULL anyway, dropping the only pointer
    to the 8KB packet buffer.  Each junk-id packet from a peer leaked.
   
    nettool's parallel implementation in src/nettool/nettool.cc has
    the identical leak in the same shape: switch falls through
    default with p still live, the caller (network_check_activity)
    doesn't free it.  nettool only constructs nwc_service_t, so
    every other reply from the server hits default and leaks 8KB
    per packet — a malicious or buggy server can starve a
    long-running admin tool of memory.

[prissi]

That could explain why the server always run out of memory after some time. Well spotted.