KeyboardInterrupt inside dict key's __str__ method leads to segmentation fault

[JustAnotherArchivist]

Reported by @smason in #419, filing as a separate issue so it doesn't get lost after that issue is fixed:

While debugging this I also noticed that if I hit Ctl+C in the above loop Python would sometimes get terminated with SIGSEGV . I can reliably reproduce this via:

class Obj:
    def __str__(self):
        raise KeyboardInterrupt

dumps({Obj(): 1})

I can still reproduce this with ujson 5.2.0, Python 3.10.1 on Debian Sid and also with #521 applied.

[JustAnotherArchivist]

There's an error check missing for PyObject_Str in Dict_iterNext. itemName becomes NULL when an exception is raised during the string conversion, and from there it blows up.