Fix boneheaded dictionary allocation oversight

UnopenableArchives was lazy-loaded, but then we went and accessed the property every time we opened a zip entry, thus constructing it and negating the whole point of lazyh loading. Now we access it only if it's been constructed.

AL_Common/FastZipReader/ZipArchiveFast.cs

@@ -225,7 +225,7 @@ private bool IsOpenable(ZipArchiveFastEntry entry, out string message)
     {
         message = "";
 
-        if (UnopenableArchives.TryGetValue(entry, out string result))
+        if (_unopenableArchives != null && _unopenableArchives.TryGetValue(entry, out string result))
         {
             message = result;
             return false;