key destructor is not correct

[shintaro-iwasaki]

The following is not implemented:

An optional destructor function destructor() may be associated with newkey. When a work unit is freed, if a key has a non-NULL destructor and the work unit has a non-NULL value associated with that key, the value of the key is set to NULL, and then destructor() is called with the last associated value as its sole argument value. The destructor is called before the work unit is freed. The order of destructor calls is undefined if more than one destructor exists for a work unit when it is freed.