ovsdb: trigger: Silence dereference after null check warning.

ovsdb: trigger: Silence dereference after null check warning.

Coverity reported a possible null dereference of t->reply
when handling errors in the "committing" state for "transact" requests.

In practice, t->reply is always non-NULL at this point, but static analysis
cannot infer this guarantee. Adding ovs_assert(t->reply) documents the
invariant for developers and ensures debug builds will catch any misuse.

No change in runtime behavior; this only improves code safety and
satisfies static analysis.

Acked-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Mike Pattrick <mkp@redhat.com>
Signed-off-by: Eelco Chaudron <echaudro@redhat.com>