array: reverse empty arrays correctly

vlib/builtin/array.v

@@ -328,6 +328,9 @@ pub fn (a mut array) push_many(val voidptr, size int) {
 // array.reverse returns a new array with the elements of
 // the original array in reverse order.
 pub fn (a array) reverse() array {
+	if a.len < 2 {
+		return a
+	}
 	arr := array{
 		len: a.len
 		cap: a.cap