This should fix a crash, as detected by Valgrind:
  ==28354== Invalid read of size 2
  ==28354==    at 0x4C2B5B0: memcpy@@GLIBC_2.14 (in
    /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
  ==28354==    by 0x50C17E2: stun_message_id (stunmessage.c:658)
  ==28354==    by 0x509E4E7: candidate_check_pair_fail (conncheck.c:254)
  ==28354==    by 0x50A4EDB: conn_check_prune_socket (conncheck.c:3145)
  ==28354==    by 0x509B6F8: component_io_cb (agent.c:3951)