assert: improve error check

Minor performance improvement.

PR-URL: #17574
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/internal/errors.js

@@ -219,10 +219,14 @@ class AssertionError extends Error {
         red = '\u001b[31m';
       }
       const util = lazyUtil();
-      if (actual && actual.stack && actual instanceof Error)
+      if (typeof actual === 'object' && actual !== null &&
+        'stack' in actual && actual instanceof Error) {
         actual = `${actual.name}: ${actual.message}`;
-      if (expected && expected.stack && expected instanceof Error)
+      }
+      if (typeof expected === 'object' && expected !== null &&
+        'stack' in expected && expected instanceof Error) {
         expected = `${expected.name}: ${expected.message}`;
+      }
 
       if (errorDiff === 0) {
         let res = util.inspect(actual);