Ensure that the shadow helper function is passed a valid property (PR 14152 follow-up)

Trying to shadow a non-existent property is always an implementation mistake, since it leads to the `shadow`-call not having any effect.

In PR 14152 I overlooked the fact that it's fairly easy to enforce this during development/testing, since that can help catch e.g. simple spelling bugs.

src/shared/util.js

@@ -488,6 +488,15 @@ function createValidAbsoluteUrl(url, baseUrl = null, options = null) {
 }
 
 function shadow(obj, prop, value) {
+  if (
+    typeof PDFJSDev === "undefined" ||
+    PDFJSDev.test("!PRODUCTION || TESTING")
+  ) {
+    assert(
+      prop in obj,
+      `shadow: Property "${prop && prop.toString()}" not found in object.`
+    );
+  }
   Object.defineProperty(obj, prop, {
     value,
     enumerable: true,