Fix the buffer handling logic so that write_impl is always called with

Fix the buffer handling logic so that write_impl is always called with
a full buffer, rather than often being called with a
slightly-less-than-full buffer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78907 91177308-0d34-0410-b5e6-96231b3b80d8