array: reverse empty arrays correctly

vlib/builtin/array_test.v

@@ -266,6 +266,10 @@ fn test_reverse() {
 	for i, _ in d {
 		assert d[i] == b[b.len - i - 1]
 	}
+
+	e := []int
+	f := e.reverse()
+	assert f.len == 0
 }
 
 const (