lib: fix "use of uninitialised value" valgrind warning

lib: fix "use of uninitialised value" valgrind warning

When FRR is built without the --enable-config-rollbacks option,
the nb_db_transaction_save() function does nothing and the
"transaction_id" output parameter is left uninitialized. For
this reason, all northbound clients should initialize the
"transaction_id" argument before calling nb_candidate_commit() or
nb_candidate_commit_apply() (except when a NULL pointer is given,
which is the case of the confd and sysrepo plugins).

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>