setting/checking init 0/NULL values in util_errmap.c

In our bug pool, there is a fixed bug related to util_errmap.c. It's still applicable to current MIT code:

mecherror_copy() needs to check values before doing copy.

In gssint_mecherrmap_map(), struct mecherror me, me_copy needs to be initialized.

I hope I can get some review feedback on free_one() and the assignment me.mech = *oid changes. I am not sure whether these two parts are still necessary. Thanks!