fix object unlink error handling

Fix the object unlink error handling so that each function cleans up
after itself properly.

Also use find_symbol() instead of __symbol_get() to make cleanup easier.
When patching a module we don't need a reference to each symbol, since
we already have done a try_module_get() on the module.

Fixes #392.
I guess this is a copy/paste from __symbol_put() so... ok