When running integration tests on macOS, we get a panic from a nil
pointer dereference when calling `ci.creds.PID()`.
This panic occurs because the `ci.creds != nil` check is insufficient
after a recent refactoring (c45f881) that changed `ci.creds` from a
pointer to the `PeerCreds` interface. Now `ci.creds` always compares as
non-nil, so we enter this block even when the underlying value is nil.
The integration tests fail on macOS when `peercred.Get()` returns the
error `unix.GetsockoptInt: socket is not connected`. This error isn't
new, and the previous code was ignoring it correctly.
Since we trust that `peercred` returns either a usable value or an error,
checking for a nil error is a sufficient and correct gate to prevent the
method call and avoid the panic.
Fixes#17421
Signed-off-by: Alex Chan <alexc@tailscale.com>
