ofproto-dpif: Reject partial ct_labels if unsupported.

ofproto-dpif: Reject partial ct_labels if unsupported.

If only half of a ct_label is present in a miniflow/minimask (eg, only
matching on one specific bit), then rule_check() would allow the flow
even if ct_label was unsupported, because it required both 64-bit fields
that comprise the ct_label to be present in the miniflow before
performing the check.

Fix this by populating the stack copy of the label directly from the
miniflow fields if available (or zero each 64-bit word if unavailable).

Suggested-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Jarno Rajahalme <jarno@ovn.org>