crypto/subtle: note that input length mismatch makes ConstantTimeCompare return immediately

Change-Id: Id1ae6c8fbb8c2f31b251ba141dc2bbedae189006
Reviewed-on: https://go-review.googlesource.com/c/go/+/316169
Trust: Daniel Martí <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Trust: Johan Brandhorst-Satzkorn <[email protected]>

Author: kortschak

src/crypto/subtle/constant_time.go

@@ -8,7 +8,8 @@ package subtle
 
 // ConstantTimeCompare returns 1 if the two slices, x and y, have equal contents
 // and 0 otherwise. The time taken is a function of the length of the slices and
-// is independent of the contents.
+// is independent of the contents. If the lengths of x and y do not match it
+// returns 0 immediately.
 func ConstantTimeCompare(x, y []byte) int {
 	if len(x) != len(y) {
 		return 0