txn: fix txn::sub_stmt_begin array size

We may write to txn->sub_stmt_begin[TXN_SUB_STMT_MAX] so the array size
must be TXN_SUB_STMT_MAX+1 (see txn_begin_stmt). This didn't lead to any
problems, because we would only overwrite txn::signature, which wouldn't
break anything. However, should we change the txn struct, we could get
an unexpected error or even a crash.

src/box/txn.h

@@ -180,7 +180,7 @@ struct txn {
 	 * First statement at each statement-level.
 	 * Needed to rollback sub statements.
 	 */
-	struct stailq_entry *sub_stmt_begin[TXN_SUB_STMT_MAX];
+	struct stailq_entry *sub_stmt_begin[TXN_SUB_STMT_MAX + 1];
 	/** LSN of this transaction when written to WAL. */
 	int64_t signature;
 	/** Engine involved in multi-statement transaction. */