w32: Fix closing file descriptors.
* src/w32-io.c (writer): Only stop once the buffer is drained. (destroy_writer): Wait for the writers buffer to be drained. This aligns '_gpgme_io_close's behavior with close(2) and fclose(3).
- GnuPG-bug-id: T2881
- Signed-off-by: Justus Winter <justus@g10code.com>