char: remove use-after-free on win-stdio

Found by reviewing the code, win_stdio_close() is called by
qemu_chr_free() which then call qemu_chr_free_common() taking care of
freeing CharDriverState*.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022095318.17775-3-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
