Bug#34929814 Double processing of FTS index words with innodb_optimize_fulltext_only

Symptom:
When running OPTIMIZE TABLE on a table with FTS index with
innodb_optimize_fulltext_only option enabled, the last word to be
processed is very rarely and apparently randomly processed twice.

Root cause:
When zlib uncompressed the end of a compressed stream, inflate() will
either return Z_STREAM_END, or it will return Z_OK, in which case the
subsequent call will output 0 bytes and return Z_STREAM_END.
fts_zip_read_word function handles the first case correctly, but in the
second the behavior is incorrect: on the call, where Z_STREAM_END is
returned from inflate(), fts_zip_read_word returns true without actually
touching the word buffer. When this happens, the caller interprets this
as the function having read the same word as it read on the previous
call, and processes that word again.

Fix:
Change the logic of the function to return false if inflate() returns
Z_STREAM_END without reading any bytes. refactor to make explicit the
various states when reading a word from the compressed stream.

Change-Id: Ib777a03b5dac0fc8bda2022b0342177f1c7d2e4a

Author: Andrzej Jarzabek