Guard the chunked body-size check against signed overflow

readRequestBodyChunked() checked the body limit with
isBodySizeAllowed(byteAmount() + currentChunkSize). Both are signed
64-bit and currentChunkSize can reach INT64_MAX, so the sum can wrap
negative, pass the limit >= size test, and let the body stream in
unbounded until OOM. The overflow is also undefined behaviour.

Use qAddOverflow() and reject on overflow.

Fixes: QTBUG-150274
Pick-to: 6.12 6.12.0 6.11
Change-Id: Ieb21816aa7f8f5e94d0302513f3a2a7b57d7cfb3
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>