dyndns_test: Use right socket length of for IPv4 address.
    
    man inet_ntop says:
      The caller specifies the number of bytes available in this buffer
      in the argument size.
    
       AF_INET
              src points to a struct in_addr (in network byte order) which is
              converted to an IPv4 network address in the dotted-decimal
              format, "ddd.ddd.ddd.ddd". The buffer dst must be at least
              INET_ADDRSTRLEN bytes long.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>