datapath: Avoid freeing wild pointer in corner case.

In odp_flow_cmd_new_or_set(), if flow_actions_alloc() fails in the "new
flow" case, then flow_put() will kfree() the new flow's 'sf_acts' pointer,
but nothing has initialized that pointer.  Initialize the pointer to NULL
to avoid the problem.

Found by inspection.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>