unicode/utf8: use builtin max function to simplify code

Change-Id: I6a73b645d074baaa4d09480bdf4192816a8c2450
GitHub-Last-Rev: 202d498
GitHub-Pull-Request: #71945
Reviewed-on: https://go-review.googlesource.com/c/go/+/652177
Auto-Submit: Keith Randall <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: callthingsoff

Diff for: src/unicode/utf8/utf8.go

@@ -263,10 +263,7 @@ func DecodeLastRune(p []byte) (r rune, size int) {
 	// guard against O(n^2) behavior when traversing
 	// backwards through strings with long sequences of
 	// invalid UTF-8.
-	lim := end - UTFMax
-	if lim < 0 {
-		lim = 0
-	}
+	lim := max(end - UTFMax, 0)
 	for start--; start >= lim; start-- {
 		if RuneStart(p[start]) {
 			break
@@ -303,10 +300,7 @@ func DecodeLastRuneInString(s string) (r rune, size int) {
 	// guard against O(n^2) behavior when traversing
 	// backwards through strings with long sequences of
 	// invalid UTF-8.
-	lim := end - UTFMax
-	if lim < 0 {
-		lim = 0
-	}
+	lim := max(end - UTFMax, 0)
 	for start--; start >= lim; start-- {
 		if RuneStart(s[start]) {
 			break