fix fwprintf missing output to open_wmemstream FILEs

fix fwprintf missing output to open_wmemstream FILEs

open_wmemstream's write method was written assuming no buffering,
since it sets the FILE up with buf_len of zero in order to avoid
issues with position/seeking. however, as a consequence of commit
bd57e2b43a5b56c00a82adbde0e33e5820c81164, a FILE being written to by
the printf core has a temporary local buffer for the duration of the
operation if it was unbuffered to begin with. since this was
disregarded by the wide memstream's write method, output produced
through this code path, particularly numeric fields, was missing from
the output wchar buffer.

copy the equivalent logic for using the buffered data from the
byte-oriented open_memstream.