Update inet_aton() to support an undocumented feature of inet_aton,

Update inet_aton() to support an undocumented feature of inet_aton,
per UNIX Network Programming, Volume 1, second edition:

    An undocumented feature of inet_aton is that if addrptr is
    a null pointer, the function still performs it validation
    of the input string, but does not store the result.