Fix a case when a pointer might be used after being freed in the ALTER TABLE code. Fix for [4722bdab08cb1].

FossilOrigin-Name: d09f8c3621d5f7f8c6d99d7d82bcaa8421855b3f470bea2b26c858106382b906

src/alter.c

@@ -755,6 +755,21 @@ static void renameWalkWith(Walker *pWalker, Select *pSelect){
   }
 }
 
+/*
+** Unmap all tokens in the IdList object passed as the second argument.
+*/
+static void unmapColumnIdlistNames(
+  Parse *pParse,
+  IdList *pIdList
+){
+  if( pIdList ){
+    int ii;
+    for(ii=0; ii<pIdList->nId; ii++){
+      sqlite3RenameTokenRemap(pParse, 0, (void*)pIdList->a[ii].zName);
+    }
+  }
+}
+
 /*
 ** Walker callback used by sqlite3RenameExprUnmap().
 */
@@ -776,6 +791,7 @@ static int renameUnmapSelectCb(Walker *pWalker, Select *p){
     for(i=0; i<pSrc->nSrc; i++){
       sqlite3RenameTokenRemap(pParse, 0, (void*)pSrc->a[i].zName);
       if( sqlite3WalkExpr(pWalker, pSrc->a[i].pOn) ) return WRC_Abort;
+      unmapColumnIdlistNames(pParse, pSrc->a[i].pUsing);
     }
   }
 
@@ -984,6 +1000,7 @@ static void renameColumnIdlistNames(
   }
 }
 
+
 /*
 ** Parse the SQL statement zSql using Parse object (*p). The Parse object
 ** is initialized by this function before it is used.