mem: check existence of parent images before dumping pages

When we are doing pre-dump, we splice pages in pipes and only then open
images and dump pages. But when we are splicing pages, we need to know
about existence of parent images. This patch adds a new call to determin
existence of parent images.

In addition this patch fixes a following issue:
CID 83244 (#1 of 1): Uninitialized pointer read (UNINIT)
14. uninit_use: Using uninitialized value xfer.parent.