ofproto: Make taking rule reference conditional on lookup.

Prior to this paths the rule lookup functions have always taken a
reference on the found rule before returning.  Make this conditional,
so that unnecessary refs/unrefs can be avoided in a later patch.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>