In the case of a stream with no filters, php_stream_fill_read_buffer
only reads stream->chunk_size into the read buffer. If the stream has
filters attached, it could unnecessarily buffer a large amount of data.
With this change, php_stream_fill_read_buffer only proceeds until either
the requested size or stream->chunk_size is available in the read buffer.
Co-authored-by: Christoph M. Becker <cmbecker69@gmx.de>
Closes GH-6444.
