ofproto: Initialise return value of modify_flows__

dd51dae29bccca3 ("ofproto: Move logic for collecting read-only rules into
rule_criteria.") modifies modify_flows__ such that the variable error,
the return value, may be uninitialised if either of the following is true:

1. ofproto->ofproto_class->rule_premodify_actions is NULL
2. rules->n is zero

It appears for the "bfd - Verify tunnel down detection" test
in the testsuite the first condition is true and the test fails.

This commit fixes the problem.

Signed-off-by: Simon Horman <horms@verge.net.au>
[blp@nicira.com changed the style of the fix]
Signed-off-by: Ben Pfaff <blp@nicira.com>