internal/routebsd: fix parsing network address of length zero

internal/routebsd: fix parsing network address of length zero

This applies CL 646555 from the net repository to this copy.

For #70528

Change-Id: Ib7e23accfa3f278392e7bdca6f8544b8f1395e7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/646676
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
TryBot-Bypass: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>