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.

----

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

JSON changes look good

Out of curiosity is there a guide you can share for running this with valgrind? I’ve tried unsuccessfully in the past but would be interested in knowing how There is no secret, in this case I did not even to bother debug builds. The only thing to remember nowadays is to use `PYTHONMALLOC=malloc` as an environment variable, otherwise you will get thousands of false positives (and generally less useful output). OK, this (unsurprisingly) did not change anything with the test, so removed it from the PR, I guess it should be all set, considering that it is a tiny bug fix really? Travis fail is unrelated, has been affecting a bunch of tests for the last few days thanks @seberg 