criu: DECLARE_ARG should check for null arguments
criu: DECLARE_ARG should check for null arguments

This is in preparation for the cgroups creation work, but also probably just a
good idea in general. The ERROR message is handy since we print line nos. it
will to give people an indication of what arg was null.

Signed-off-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>
Acked-by: Serge E. Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

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