Silence compiler warning in Init_New_Client()

Silence compiler warning in Init_New_Client()

Use strdup() instead of pointless strndup() to fix the following
compiler warning:

client.c: In function ‘Init_New_Client’:
client.c:216:32: warning: ‘strndup’ specified bound 127 exceeds source size 5 [-Wstringop-overread]
  216 |                 client->away = strndup(DEFAULT_AWAY_MSG, CLIENT_AWAY_LEN - 1);
      |                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~