Improve error handling in OBJ_add_object()

Improve error handling in OBJ_add_object()

There is no need for a helper function to obfuscate lh_ADDED_OBJ_new().
Just call the real thing directly.

Adding an object with a NID of NID_undef basically amounts to disabling
a built-in OID. It does so in an incoherent fashion and the caller can't
easily tell success from failure of the operation. Arguably the result is
a corrupted objects table.

Let's not allow adding such an object in an attempt at keeping things
slightly more coherent.

Issue noted and initial diff by schwarze while writing documentation

ok schwarze