auto merge of #12396 : alexcrichton/rust/windows-env-var, r=huonw

On windows, the GetEnvironmentVariable function will return the necessary buffer
size if the buffer provided was too small. This case previously fell through the
checks inside of fill_utf16_buf_and_decode, tripping an assertion in the `slice`
method.

This adds an extra case for when the return value is >= the buffer size, in
which case we assume the return value as the new buffer size and try again.

Closes #12376

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