Ensure that the Expr objects that describe indexed expressions are not modified
by code generation.  Fix for an assert() problem found by Jon Metzman using AFL.

FossilOrigin-Name: 34073ce87d88a02313217023ae92e15939192cd9
diff --git a/src/insert.c b/src/insert.c
index 52769e9..7ac2a25 100644
--- a/src/insert.c
+++ b/src/insert.c
@@ -1408,7 +1408,7 @@
       int x;
       if( iField==XN_EXPR ){
         pParse->ckBase = regNewData+1;
-        sqlite3ExprCode(pParse, pIdx->aColExpr->a[i].pExpr, regIdx+i);
+        sqlite3ExprCodeCopy(pParse, pIdx->aColExpr->a[i].pExpr, regIdx+i);
         pParse->ckBase = 0;
         VdbeComment((v, "%s column %d", pIdx->zName, i));
       }else{