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/sqliteInt.h b/src/sqliteInt.h
index 4249ef7..a328aff 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -3461,6 +3461,7 @@
 void sqlite3ExprCacheClear(Parse*);
 void sqlite3ExprCacheAffinityChange(Parse*, int, int);
 void sqlite3ExprCode(Parse*, Expr*, int);
+void sqlite3ExprCodeCopy(Parse*, Expr*, int);
 void sqlite3ExprCodeFactorable(Parse*, Expr*, int);
 void sqlite3ExprCodeAtInit(Parse*, Expr*, int, u8);
 int sqlite3ExprCodeTemp(Parse*, Expr*, int*);