internal/fuzz: use the built-in min function

internal/fuzz: use the built-in min function

Change-Id: I26be9a9faa27c6c03cf8b9dec5908d8617d61312
Reviewed-on: https://go-review.googlesource.com/c/go/+/518276
Run-TryBot: shuang cui <imcusg@gmail.com>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>