Commit 5defdc38
(which aimed to reduce sizeof(mtr_t) for non-debug builds)
replaced the ternary mtr_t::status with two debug-only bool
data members m_start, m_commit and inadvertently made the
(now debug-only) predicate mtr_t::is_active() wrongly hold
after mtr_t::commit().

mtr_t::is_active(): Evaluate both m_start and m_commit,
to be compatible with the old definition.

row_merge_read_clustered_index(): Correct a debug assertion.