MDEV-22332: Assertion mtr_started == mtr.is_active() failed

Commit 5defdc382bbf606b83e556c4f0d29dcd7954ebbc
(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.

storage/innobase/include/mtr0mtr.h

@@ -614,7 +614,8 @@ private:
 #ifdef UNIV_DEBUG
 public:
   /** @return whether the mini-transaction is active */
-  bool is_active() const { ut_ad(!m_commit || m_start); return m_start; }
+  bool is_active() const
+  { ut_ad(!m_commit || m_start); return m_start && !m_commit; }
   /** @return whether the mini-transaction has been committed */
   bool has_committed() const { ut_ad(!m_commit || m_start); return m_commit; }
 private:

storage/innobase/row/row0merge.cc

@@ -1984,7 +1984,7 @@ row_merge_read_clustered_index(
 				os_thread_yield();
 scan_next:
 				ut_ad(!mtr_started);
-				ut_ad(mtr.is_active());
+				ut_ad(!mtr.is_active());
 				mtr.start();
 				mtr_started = true;
 				/* Restore position on the record, or its