Instead of *always* resetting the current_config to null, we should only
reset it if this API call set it.

This allows nesting of API calls, e.g. c->checkpoint() can pass stuff into
criu.c, which can call c->init_pid() and not lose the ability to log stuff
afterwards.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>