que_run_threads(): Check we haven't accidentally left an active transaction

in e.g. TRX_LOCK_WAIT state.

que/que0que.c

@@ -1347,6 +1347,14 @@ que_run_threads(
 		que_thr_dec_refer_count(thr, &next_thr);
 
 		if (next_thr == NULL) {
+			trx_t*	trx = thr_get_trx(thr);
+
+			if (trx->conc_state == TRX_ACTIVE) {
+				/* Check we haven't accidentally left the
+				transaction in TRX_LOCK_WAIT or something. */
+
+				ut_a(trx->que_state == TRX_QUE_RUNNING);
+			}
 
 			return;
 		}