A fix for Bug#5748 "Prepared statement with BETWEEN and bigint values

crashes mysqld": implementation for a generic item tree modifications
registry. Every item tree modification which should be rolled back for
subsequent execution of a prepared statement or stored procedure should
be saved in the registry. All such modifications are rolled back at once
during cleanup stage of PS.
Actual fix for the bug just adds a call to register modifications to
convert_constant_item.
Post review fixes implemented.

sql/sql_parse.cc

@@ -4056,6 +4056,7 @@ void mysql_parse(THD *thd, char *inBuf, uint length)
     }
     thd->proc_info="freeing items";
     thd->end_statement();
+    DBUG_ASSERT(thd->change_list.is_empty());
   }
   DBUG_VOID_RETURN;
 }