Fix reporting errors with gai_strerror(..)

The return value (err) should be checked; not the errno value.

PR: 235200
Approved by: emaste (mentor)
Reviewed by: asomers, lwhsu
MFC after: 28 days
MFC with: r343362, r343365, r343367-r343368
Differential Revision: https://reviews.freebsd.org/D18969