When generating code for a subquery, make a copy of the Select object and

generate the code out of the copy, in case the code generator makes
modifications to expression and the Select object needs to be reused.

FossilOrigin-Name: 4edddcc0bc8d71e9b8abac67bc3766f1d9143dddd1f59264859ce65e5aa9b8c6

test/select1.test

@@ -1184,5 +1184,18 @@ do_catchsql_test select-19.21 {
     ORDER BY 1;
 } {1 {table t1 has 1 columns but 15 values were supplied}}
 
+# 2020-01-01 Found by Yongheng's fuzzer
+#
+reset_db
+do_execsql_test select-20.10 {
+  CREATE TABLE t1 (
+    a INTEGER PRIMARY KEY,
+    b AS('Y') UNIQUE
+  );
+  INSERT INTO t1(a) VALUES (10);
+  SELECT * FROM t1 JOIN t1 USING(a,b)
+   WHERE ((SELECT t1.a FROM t1 AS x GROUP BY b) AND b=0)
+      OR a = 10;
+} {10 Y}
 
 finish_test