BUG: Fix webp encode errors on win-amd64

This PR fixes two webp related test failures on win-amd64 (below). The uninitialized variable `size` of type  `Py_ssize_t`  should not be parsed as `int`.

```
<snip>
running test_file_webp ...
Tests\test_file_webp.py:43: lena("RGB").save(temp_file) failed:
Traceback (most recent call last):
  File "Tests\test_file_webp.py", line 43, in test_write_rgb
    lena("RGB").save(temp_file)
  File ".\PIL\Image.py", line 1465, in save
    save_handler(self, fp, filename)
  File ".\PIL\WebPImagePlugin.py", line 52, in _save
    fp.write(data)
TypeError: 'NoneType' does not support the buffer interface
Tests\test_file_webp.py:79: pil_image.save(temp_file) failed:
Traceback (most recent call last):
  File "Tests\test_file_webp.py", line 79, in test_write_rgba
    pil_image.save(temp_file)
  File ".\PIL\Image.py", line 1465, in save
    save_handler(self, fp, filename)
  File ".\PIL\WebPImagePlugin.py", line 52, in _save
    fp.write(data)
TypeError: 'NoneType' does not support the buffer interface
<snip>
```