Fix bogus error checking on unique_fd

The expression "!fd" calls the implicit conversion to int, but comparing
the raw fd against 0 does not work, since open() and other POSIX calls
returning a file descriptor use -1 to signal an error.

Test: m libnetd_resolv
Test: sync, then atest resolver_integration_test
Change-Id: I876fe64a5507a0863151b6807990fdfa72e2973e