Fix #113400: USD transform cache crash on invalid prim.

Updated CacheReader_open_usd_object() to issue a warning and
return early if the given object path does not refer to a
valid prim on the USD stage, to avoid crashing when attempting
to create a reader from an invalid prim.

Pull Request: https://projects.blender.org/blender/blender/pulls/113524

source/blender/io/usd/intern/usd_capi_import.cc

@@ -553,6 +553,11 @@ CacheReader *CacheReader_open_usd_object(CacheArchiveHandle *handle,
 
   pxr::UsdPrim prim = archive->stage()->GetPrimAtPath(pxr::SdfPath(object_path));
 
+  if (!prim) {
+    WM_reportf(RPT_WARNING, "USD Import: unable to open cache reader for object %s", object_path);
+    return nullptr;
+  }
+
   if (reader) {
     USD_CacheReader_free(reader);
   }