Fixed client buffer reallocation for HTTP/3. Preserving pointers within the client buffer is not needed for HTTP/3 because all data is either allocated from pool or static. Unlike with HTTP/1, data typically cannot be referenced directly within the client buffer. Trying to preserve NULLs or external pointers lead to broken pointers. Also, reverted changes in ngx_http_alloc_large_header_buffer() not relevant for HTTP/3 to minimize diff to mainstream.