fix noise log when fallthrough from gofail's if block

[lysu]

how to reproduce

what I do

import "fmt"

func Test() string {
	// gofail: var beFail bool
	// if beFail {
	// 	fmt.Print(1)
	// }
	fmt.Println(2)
	return "ok"
}

and enable gofail and return false by

import (
	gofail "github.com/etcd-io/gofail/runtime"
)

func TestGoFail(t *testing.T) {
	gofail.Enable("github.com/lysu/go-misc/fail/beFail", "return(false)")
	defer gofail.Disable("github.com/lysu/go-misc/fail/beFail")
	t.Log(fail.Test())
}

expect

print 2 and return ok

but got

std log message

failpoint: "github.com/lysu/go-misc/fail/beFail" got value false of type "bool" but expected type "bool"

and print 2 and return ok

what question

generate code after gofail enable will be

func Test() string {
	if vbeFail, __fpErr := __fp_beFail.Acquire(); __fpErr == nil { defer __fp_beFail.Release(); beFail, __fpTypeOK := vbeFail.(bool); if !__fpTypeOK { goto __badTypebeFail}
		if beFail {
			fmt.Print(1)
		}; __badTypebeFail: __fp_beFail.BadType(vbeFail, "bool"); };
	fmt.Println(2)
	return "ok"
}

when beFail as false, it will continue execute __fp_beFail.BadType(vbeFail, "bool"); event if there are no type error.

what I fix

to generate this code:

func Test() string {
	if vbeFail, __fpErr := __fp_beFail.Acquire(); __fpErr == nil { defer __fp_beFail.Release(); beFail, __fpTypeOK := vbeFail.(bool); if !__fpTypeOK { goto __badTypebeFail}
		if beFail {
			fmt.Print(1)
		}; goto __nomockbeFail; __badTypebeFail: __fp_beFail.BadType(vbeFail, "bool"); __nomockbeFail: };
	fmt.Println(2)
	return "ok"
}

[ahrtr]

Sorry for the late response.

This PR looks good to me. Could you please rebase it?