Treat Z_BUF_ERROR as TOR_ZLIB_BUF_FULL when finalizing a zlib buffer

Otherwise, when we're out of input *and* finalizing, we might report
TOR_ZLIB_OK erroneously and not finalize the buffer.

(I don't believe this can happen in practice, with our code today:
write_to_buf_zlib ensures that we are never trying to write into a
completely empty buffer, and zlib says "Z_OK" if you give it even
one byte to write into.)

Fixes bug 11824; bugfix on 0.1.1.23 (06e09cdd47eaa8f7e90ce2).
Merge remote-tracking branch 'public/bug11824_v2'
Fix a changes typo spotted by wfn