ovs-ofctl: Fix del-flows command parsing bugs.

ovs-ofctl: Fix del-flows command parsing bugs.

"ovs-ofctl del-flows br0" segfaulted because do_flow_mod__() assumed that
it always had a "flow" argument, which is not true for the del-flows
command.

Beyond that, parse_ofp_flow_mod_str() rejected "ovs-ofctl del-flows
br0" because no actions were supplied, even though supplying actions
doesn't make sense for deleting flows.

This commit fixes both problems and adds a simple test that would have
caught both problems.

Bug #4112.