Fix FK_table_backup removed

Tests affected: innodb-index

sql/sql_alter.h

@@ -402,6 +402,7 @@ class Alter_table_ctx
   */
   FK_share_backup* fk_add_backup(TABLE_SHARE *share)
   {
+    // NB: FK_share_backup ctor is used, ownership is still fk_shares
     return fk_ref_backup.emplace(NULL, share, share);
   }
   bool fk_install_frms();

sql/sql_table.cc

@@ -8486,6 +8486,9 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
   }
   DBUG_ENTER("mysql_prepare_alter_table");
 
+  if (!alter_ctx->fk_add_backup(table->s))
+    DBUG_RETURN(1);
+
   /*
     Merge incompatible changes flag in case of upgrade of a table from an
     old MariaDB or MySQL version.  This ensures that we don't try to do an