gethostbyaddr_r: set result pointer to null on error, per specification

[AllSeeingEyeTolledEweSew]

gethostbyaddr_r should set its struct hostent **result parameter to NULL on an error (reference). torsocks doesn't do this currently.

python, for example, gets tripped up by this:

$ torsocks python3 -c 'import socket; socket.getfqdn("127.0.0.1")'
1607419174 ERROR torsocks[3239]: Unable to resolve. Status reply: 4 (in socks5_recv_resolve_ptr_reply() at socks5.c:823)
Segmentation fault