[dev.fuzz] internal/fuzz: add sharedMem.setValueLen

[dev.fuzz] internal/fuzz: add sharedMem.setValueLen

This method sets the len of the slice returned by valueRef. The worker
now uses this instead of setting the length in the header directly.

Unfortunately, we can't store the whole slice header in the shared
memory header because the pointer won't be valid across processes.

Change-Id: Icef24acfcd85e098cd8c23810568f04b13649a19
Reviewed-on: https://go-review.googlesource.com/c/go/+/284012
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>