In r268924 __fflush was modified so that when write(2) was not successful,
_p and _w are adjusted to account for the partial write (if any).

However, _p and _w should not be unconditionally adjusted and should only
be changed when we actually wrote some bytes, or the accumulated accounting
error will eventually result in a heap buffer overflow.

Reported by: adrian and alfred (Norse Corporation)
Security: FreeBSD-SA-14:27.stdio
Security: CVE-2014-8611