Range filter: fixed loss of incoming chain links. Filters are not allowed to change incoming chain links, and should allocate their own links if any modifications are needed. Nevertheless ngx_http_range_singlepart_body() modified incoming chain links in some cases, notably at the end of the requested range. No problems caused by this are currently known, mostly because of limited number of possible modifications and the position of the range body filter in the filter chain. Though this behaviour is clearly incorrect and tests demonstrate that it can at least cause some proxy buffers being lost when using proxy_force_ranges, leading to less effective handling of responses. Fix is to always allocate new chain links in ngx_http_range_singlepart_body(). Links are explicitly freed to ensure constant memory usage with long-lived requests.