Alembic: Fix double-free of mutex

The mutex was shared between CoW copies of the CacheFile datablock, and
as a result also freed multiple times. It is now only freed when the
original datablock is freed; the CoW copies share the same mutex.

source/blender/blenkernel/intern/cachefile.c

@@ -93,7 +93,9 @@ void BKE_cachefile_free(CacheFile *cache_file)
 	ABC_free_handle(cache_file->handle);
 #endif
 
-	if (cache_file->handle_mutex) {
+	/* CoW copies share the mutex, so it should only be freed if the original
+	 * CacheFile datablock is freed. */
+	if (cache_file->handle_mutex && (cache_file->id.tag & LIB_TAG_COPIED_ON_WRITE) == 0) {
 		BLI_mutex_free(cache_file->handle_mutex);
 	}
 	BLI_freelistN(&cache_file->object_paths);