dpif-netdev: Tolerate undersized packets.

dpif-netdev: Tolerate undersized packets.

Actions that modify packets need to tolerate packets that are too small.
Most of the actions already implicitly do this check, since they check for
appropriate values in the flow key that would only be there if the
corresponding data was present.  But actions to modify the Ethernet header
didn't have a guarantee that the packet was at least 14 bytes long, and
actions to modify the VLAN didn't have such a guarantee either, so this
adds appropriate checks.

Problem found by code inspection.