In the upstream google/brotli, if the `decompress` function receives an empty byte
string, it returns a brotli.error; whereas in 'brotlipy' it does not raise but
returns an empty string b"":
https://github.com/python-hyper/brotlipy/issues/43#issuecomment-240378257
This test case asserts that when 'totalCompressedSize' in the WOFF2 header is
incorrectly set, the woff2 reader fails -- either because the brotli decoder
raises an exception, or it returns a string whose length is not the one expected.
Using TTFont.getGlyphOrder() would also require to decompile post and cmap tables.
But we don't need the actual glyph names in order to apply the WOFF2 glyf transform.
