internal/fuzz: fix chunk swap mutator

When swapping two chunks of bytes in a slice, don't pick chunks which
extend beyond the end of the slice. Also don't pick chunks which
intersect with each other.

Fixes #49047

Change-Id: I070eb1888d05ae849ec6122d01c40c45e602019f
Reviewed-on: https://go-review.googlesource.com/c/go/+/365175


Trust: Roland Shoemaker <roland@golang.org>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Katie Hockman <katie@golang.org>