Fix non-POSIX-compliant use of getaddrinfo in libfetch

[grembo]

Summary

Boris Kolpackov brought this to my attention: libfetch uses
getaddrinfo in a non-compliant way, as it assumes that
res will be NULL when getaddrinfo fails. Even this
is true in reality right now, it's neither guaranteed by
POSIX, nor by the getaddrinfo man page. To prevent future
breakage and to be a good citizen, this patch aims
to fix this.

Test Plan

Build.