Fix assert in Alembic tests with constraints

Leave the reader to be created on the CoW object in the depsgraph evaluation,
don't assign the one used for importing to the original object.

source/blender/alembic/intern/abc_object.cc

@@ -263,9 +263,6 @@ void AbcObjectReader::setupObjectTransform(const float time)
 
     data->cache_file = m_settings->cache_file;
     id_us_plus(&data->cache_file->id);
-
-    data->reader = reinterpret_cast<CacheReader *>(this);
-    this->incref();
   }
 }
 

source/blender/blenkernel/intern/cachefile.c

@@ -100,11 +100,11 @@ void BKE_cachefile_reader_open(CacheFile *cache_file,
 void BKE_cachefile_reader_free(CacheFile *cache_file, struct CacheReader **reader)
 {
 #ifdef WITH_ALEMBIC
-  if (cache_file) {
-    BLI_assert(cache_file->id.tag & LIB_TAG_COPIED_ON_WRITE);
-  }
-
   if (*reader != NULL) {
+    if (cache_file) {
+      BLI_assert(cache_file->id.tag & LIB_TAG_COPIED_ON_WRITE);
+    }
+
     CacheReader_free(*reader);
     *reader = NULL;