pagemap: init pagemap entries array as NULL

There is call to close_page_read on open_pages_image_at failure,
also on failure of init_pagemaps. pmes[] is uninitialized here
and free_pagemaps() will try to walk them and call xfree().
Which surely would lead to crash.

pagemap.c:317:6: warning: Branch condition evaluates to a garbage value
        if (pr->pmes)
            ^~~~~~~~

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>