syscall: check if to is nil to prevent panic from Sendto

syscall: check if to is nil to prevent panic from Sendto

Fixes #55845

Change-Id: If30263b7d34829c40e6606846115ec520f351f2c
Reviewed-on: https://go-review.googlesource.com/c/go/+/433504
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>