If a conflict happens under wsrep_on, the THD's wsrep_conflict_state
is typically set to MUST_ABORT and cleared later, when transaction is
aborted. However, when wsrep_on is disabled, no check is performed to
see whether wsrep_conflict_state is set. So this potentially creates
spurious deadlock errors on the subsequent statement that runs with
wsrep_on enabled.
To avoid this problem wsrep_thd_set_conflict_state() sets the conflict
state only if wsrep_on is enabled.