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>

criu/pagemap.c

@@ -436,6 +436,7 @@ int open_page_read_at(int dfd, int pid, struct page_read *pr, int pr_flags)
 	pr->pi_off = 0;
 	pr->bunch.iov_len = 0;
 	pr->bunch.iov_base = NULL;
+	pr->pmes = NULL;
 
 	pr->pmi = open_image_at(dfd, i_typ, O_RSTR, (long)pid);
 	if (!pr->pmi)