vconn: When validating OpenFlow actions always check for bad length.

vconn: When validating OpenFlow actions always check for bad length.

This code was previously unreachable, but it makes sense to check the
action length before looking at the action any further.

This doesn't fix an actual bug--actions were always properly validated.

Also, rephrase a few of the switch cases to make it even more obvious
that they always return.

Reported-by: Jean Tourrilhes <jt@hpl.hp.com>