ploop_copy_send(): fix potential NULL deref

As pointed out by Coverity, CID 1216781.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
ploop copy: fix open_mount_point err handling

Before this patch:

	# ploop -vvvv copy -s /dev/ploop47147 -d file
	[ 0.000337] Error in open_mount_point (pcopy.c:357):
		Can't open : No such file or directory

Apparently, ploop_get_mnt_by_dev() can return 1 in case there is
no error, but mount point can't be found, therefore we should check
for non-zero, not just < 0.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>