fix overflow on negative bytes (#7070)

src/tests/test_math.rs

@@ -104,3 +104,8 @@ fn floating_add() -> TestResult {
 fn precedence_of_or_groups() -> TestResult {
     run_test(r#"4 mod 3 == 0 || 5 mod 5 == 0"#, "true")
 }
+
+#[test]
+fn test_filesize_op() -> TestResult {
+    run_test("-5kb + 4kb", "-1,000 B")
+}