security: skip obtaining additional groups for root, as they don't matter

As it was reported, some systems don't use /etc/passwd.
On such systems getpwuid fails with undefined errno(see getpwuid(3))
not allowing criu to restrict ids with user additional groups.
Luckily, on such systems criu is run as root, so we can
just skip obtaining additional groups, as they don't matter
for root.

Reported-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Tested-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>