Prevent errors in Express 5.x applications (#4872)

* Fix integration by preventing unsafe access to properties.

---------

Co-authored-by: William Conti <william.conti@datadoghq.com>
Co-authored-by: William Conti <58711692+wconti27@users.noreply.github.com>

Author: wantsui

packages/datadog-instrumentations/src/router.js

@@ -112,7 +112,6 @@ function createWrapRouterMethod (name) {
       path: pattern instanceof RegExp ? `(${pattern})` : pattern,
       test: layer => {
         const matchers = layerMatchers.get(layer)
-
         return !isFastStar(layer, matchers) &&
           !isFastSlash(layer, matchers) &&
           cachedPathToRegExp(pattern).test(layer.path)
@@ -121,15 +120,15 @@ function createWrapRouterMethod (name) {
   }
 
   function isFastStar (layer, matchers) {
-    if (layer.regexp.fast_star !== undefined) {
+    if (layer.regexp?.fast_star !== undefined) {
       return layer.regexp.fast_star
     }
 
     return matchers.some(matcher => matcher.path === '*')
   }
 
   function isFastSlash (layer, matchers) {
-    if (layer.regexp.fast_slash !== undefined) {
+    if (layer.regexp?.fast_slash !== undefined) {
       return layer.regexp.fast_slash
     }