Investigate Zstandard decompression error

make test-network with current tor HEAD yields a number of warning related to Zstandard decompression, which indicates we have a bug somewhere. The relevant error snippet is:

Warning: Error while uncompresing data: bad input? Number: 2
Warning: Zstandard decompression didn't finish: Unknown frame descriptor. Number: 2

Both Nick and I are able to reproduce this issue on FC and Debian stretch with different versions of libzstd, which probably indicate that this issue is in our code.