criu: fix segfault in pre-dump

By accident I found a segfault using pre-dump in combination with the
page-server. Doing the following I was able to trigger it:

 * criu page-server -D /tmp/1
 * criu pre-dump -t PID -D /tmp/3 --track-mem
 * criu page-server -D /tmp/4 --prev-images-dir ../1
 * criu pre-dump -t PID -D /tmp/3 --track-mem
 --> segfault

...
(00.010090) Warn  (criu/image.c:134): Failed to open parent directory
...
(00.012984) Error (criu/mem.c:318): Pid-reuse detection failed: no parent inventory, check warnings in get_parent_stats
...
(00.013037) Error (criu/mem.c:544): Can't dump page with parasite
...
(00.013955) Pre-dumping tasks' memory
(00.013966) 	Pre-dumping 8793
(00.014380) Transferring pages:
Segmentation fault (core dumped)

Looking in cr-dump.c at cr_pre_dump_finish(int ret) the function gets
the return code of the previous operations in 'ret' but it is
immediately overwritten and never used.

In older CRIU versions it used to be:

	if (ret < 0)
		goto err;

but that is gone now. So this reintroduces the check for the int
parameter given to cr_pre_dump_finish() by the function caller.

As the commands used to trigged the segfault do not make much sense the
result is still not usable and the same 'Warn' and 'Error' messages are
printed, but the segfault is gone.

V2:
  * set the return value correctly

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>