Remove useless check for negative result of ip_addrsize().

Remove useless check for negative result of ip_addrsize().

By inspection, ip_addrsize() can't return a negative result.
(If it could, we'd have way bigger problems elsewhere.)
So delete useless check in network_send().  Most C compilers
are probably perfectly capable of removing this code by
themselves, but it's confusing/misleading.

Bug: #18889
Reported-by: Daniel Elishakov 
Discussion: https://postgr.es/m/18889-73d4f19e953a629e@postgresql.org