Fix Unicode support for ostream redirects (#2982)

* Crash when printing Unicode to redirected cout
Add failing tests

* Fix Unicode crashes redirected cout

* pythonbuf::utf8_remainder check end iterator

* Remove trailing whitespace and formatting iostream

* Avoid buffer overflow if ostream redirect races
This doesn't solve the actual race, but at least it now has a much lower
probability of reading past the end of the buffer even when data races
do occur.