ofp-parse: Correctly update bucket lists if they are empty.

ofp-parse: Correctly update bucket lists if they are empty.

Previously, list_moved() only worked with non-empty lists, but this was a
caveat that was really easy to miss.  parse_ofp_group_mod_file() had a bug
because it didn't honor that restriction.  This commit fixes the problem,
by modifying the list_moved() interface to be harder to use incorrectly
and then updating the callers.

Reported-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Thomas Graf <tgraf@noironetworks.com>