Bug 1719319: Make consistency error non-fatal on relase. r=mccr8

I still haven't figured out why this is happening, so since we can make it
non-fatal on release builds, we should, given the crash volume.

I don't want to completely silence the issue in non-release builds, since
something is clearly still wrong.

Differential Revision: https://phabricator.services.mozilla.com/D121686

js/xpconnect/loader/URLPreloader.cpp

@@ -304,6 +304,11 @@ Result<Ok, nsresult> URLPreloader::ReadCache(
       auto entry = mCachedURLs.GetOrInsertNew(key, key);
       entry->mResultCode = NS_ERROR_NOT_INITIALIZED;
 
+      if (entry->isInList()) {
+        MOZ_DIAGNOSTIC_ASSERT(false, "Entry should be new and not in any list");
+        return Err(NS_ERROR_UNEXPECTED);
+      }
+
       pendingURLs.insertBack(entry);
     }