Fixed client buffer reallocation for HTTP/3.

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.