ofp-actions: Properly check for action that exceeds buffer length.

Commit c2d936a44fa (ofp-actions: Centralize all OpenFlow action code for
maintainability.) rewrote OpenFlow action parsing but failed to check that
actions don't overflow their buffers.  This commit fixes the problem and
adds negative tests so that this bug doesn't recur.

Reported-by: Tomer Pearl <Tomer.Pearl@Contextream.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>