Ensure showbase does not overflow do_put buffers

Summary:
In https://bugs.freebsd.org/207918, Daniel McRobb describes how using
std::showbase with ostreams can cause truncation of unsigned long long
when output format is octal.  In fact, this can even happen with
unsigned int and unsigned long.

To ensure this does not happen, add one additional character to the
do_put buffers if std::showbase is on.  Also add a test case.

Reviewers: EricWF, mclow.lists

Reviewed By: EricWF

Subscribers: cfe-commits, emaste

Differential Revision: https://reviews.llvm.org/D32670