[GIT-PULLS] [php-src] PR #23439: Fix read buffer compaction in stream filter flush

Pull Request: https://github.com/php/php-src/pull/23439
Author: crystarm

`php_stream_filter_flush()` compacts unread data before appending buckets produced by a read filter.

The source and destination ranges may overlap, making the use of `memcpy()` undefined behavior. Additionally, `readpos` was reset before it was subtracted from `writepos`, so the buffer size was not adjusted and stale data could remain visible.

Use `memmove()` and adjust `writepos` before resetting `readpos`, matching the existing buffer compaction logic in `php_stream_fill_read_buffer()`.

The issue was detected by static analysis:  [BUFFER_OVERLAP filter.c:[458:4].log](https://github.com/user-attachments/files/31381411/BUFFER_OVERLAP.filter.c.458.4.log)