BUG: Fix small memory access errors in ujson objToJSON.c
These were found (and validated fixed) using valgrind. It is likely
that none of these changes should create any issues (except maybe
on a debug build of python?). But silencing valgrind warnings is
good on its own, to ease possible future debugging using valgrind.

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NOTE: I removed skipping of `test_astype_generic_timestamp_no_frequency`, this is just out of curiosity, I would be very surprised if it made a difference. So this should probably be only merged after undoing the test change.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
