openvswitch: Avoid assigning a NULL pointer to flow actions.

Flow SET can accept an empty set of actions, with the intended
semantics of leaving existing actions unmodified.  This seems to have
been brokin after OVS 1.7, as we have assigned the flow's actions
pointer to NULL in this case, but we never check for the NULL pointer
later on.  This patch restores the intended behavior and documents it
in the include/linux/openvswitch.h.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>