When `php_zlib_deflate_filter()` is called with `PSFS_FLAG_FLUSH_INC`
but without new buckets being available (e.g. because a user calls
`rewind()` after writing to the stream), we have to make sure that any
pending data are flushed. This could basically be done like in the
attached patch[1], but that could cause unnessary flushes, which can be
harmful for compression, and adds unnecessary flush markers to the
stream. Thus, we use the `php_zlib_filter_data.finished` field, which
has not been used for `zlib.deflate` filters, and properly keep track
of the need to flush.
[1] <https://bugs.php.net/patch-display.php?bug_id=48725&patch=zlib-filter-flush-fix.patch&revision=latest>
Closes GH-6019.
