reflect: panic if ArrayOf is called with negative length

reflect: panic if ArrayOf is called with negative length

Since we cannot change the signature of reflect.ArrayOf to return an
error, we panic instead of producing a wrong result.

Fixes #43603

Change-Id: I23915df8d190f35af4d00ab86768868cd621e839
Reviewed-on: https://go-review.googlesource.com/c/go/+/284136
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>