win,test: don't write uninitialized buffer to tty
win,test: don't write uninitialized buffer to tty

Fixes an issue where the Windows-only tty_large_write attempts to write
an uninitialized buffer to the console.

PR-URL: https://github.com/libuv/libuv/pull/1212
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>
Reviewed-By: Santiago Gimeno <3f17e29e343c4cf8809fb0970d81d6d8ee88d6b3@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso