internal/fuzz: compute correct number of mutations

When reconstructing inputs, we miscalculated the number of mutations
that needed to be applied. If the count%chainedMutation == 0 we would
apply 0 mutations, when we should actually be applying chainedMutation
mutations, due to how count is incremented.

Fixes #49047

Change-Id: I76773bff0afd6dfd40deafc317be095da995ecc5
Reviewed-on: https://go-review.googlesource.com/c/go/+/365294


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