Description of problem:

Modules/socketmodule.c contains:

#ifdef ENABLE_IPV6
        char ip[MAX(INET_ADDRSTRLEN, INET6_ADDRSTRLEN) + 1];
#else
        char ip[INET_ADDRSTRLEN + 1];
#endif
...
        /* Guarantee NUL-termination for PyString_FromString() below */
        memset((void *) &ip[0], '\0', sizeof(ip) + 1);

which overwrites the ip[] buffer by 1 byte.