ofproto: Avoid theoretical double free of large rule collections.

collect_rules_strict() and collect_rules_loose() destroy the rule
collections that they create if they return an error, and some of their
callers then go on to destroy them again.  This could cause a double-free
in the case where rule_collection_destroy() actually calls free().  That
never happens in the current tree, because free() is only necessary if
malloc() was called and there's a 64-entry stub that none of the current
code in collect_rules_*() can fill up in their error cases.  Still, it
seems better to fix the problem.

Found by clang-analyzer.

This is backported to OVS 2.1 because a discussion on ovs-discuss
showed that in fact the double free can trigger:
http://openvswitch.org/pipermail/discuss/2014-September/014978.html

Reported-by: Anup Khadka <khadka.py@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>