prevent underflow as well as overflow when adding an instance time

Differential Revision: https://phabricator.services.mozilla.com/D228495

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1930221
gecko-commit: accae6f53b74984481f38ca4efe82246665fb46e
gecko-reviewers: emilio

svg/animations/crashtests/firefox-bug-1930221.html

@@ -0,0 +1,7 @@
+<script>
+document.addEventListener('DOMContentLoaded', () => {
+  document.getElementById("x").beginElementAt(-1369430904473181976352858)
+})
+</script>
+<svg>
+<animate id="x" begin="a" />