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Yea, testing for the proper input length is definitely
something that should be done.  The patch looks good, but
for one thing.  If the specified address family is neither
AF_INET nor AF_INET6, the length won't be tested and the
underlying inet_ntop will be called.  This isn't a problem
now (afaik) because only those two address families are
support, but in a future libc version with more supported
address families, it might open a similar hole to the one
you've fixed.  Perhaps the

+       } else {
+               PyErr_SetString(socket_error, "unknown
address family");
+               return NULL;
+       }

should be moved up from the second if-grouping to follow the
first if-grouping.  Everything else looks good to me. 
Thanks for taking the time to look at this :)