bridge: Skip statistics update if cannot get ovsdb config.

Commit 62c5c3e (bridge: Rate limit the statistics update.) makes
ovs try committing the statistics even if ovs failed to get ovsdb
configuration.  This causes the ovs committing the NULL transaction
pointer.

To fix this issue, this commit makes ovs skip statistics update
if it cannot get ovsdb config.

VMware-BZ: #1331308

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>