[dev.fuzz] internal/fuzz: don't use temporary buffers

We're already allocating a huge amount of space at the end of each byte
slice, so just use that for scratch space instead of allocating a new
temporary buffer.

Change-Id: Ie075a199d4ac5dd5789949af2a19815c3025b809
Reviewed-on: https://go-review.googlesource.com/c/go/+/314272


Trust: Roland Shoemaker <roland@golang.org>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>