Previous attempt to fix reference leaks was in issue984722.

Proposed patch uses different approach. It correctly fixes reference leaks after error in processing dict items (like "{()s(())N"), fixes leaksafter memory errors on creating tuple, list or dict, fixes issue20024 for lists and dicts, fixes use-after-free issue described in msg259242.

But the patch has a disadvantage in comparison with current code. If user converter ("O&") accepts Python objects and steal its reference (as "N"), this reference will be leaked. This is a price for avoiding potential use-after-free issues. Current stdlib code doesn't use converters that steal Python object reference. May be we should document this limitation.