dnsdist-1.8.x: YaHTTP: Prevent integer overflow on very large chunks

[rgacogne]

Short description

Backport of #12892 to rel/dnsdist-1.8.x

If the chunk_size is very close to the maximum value of an integer, we trigger an integer overflow when checking if we have a trailing newline after the payload.
Reported by OSS-Fuzz as:
https://oss-fuzz.com/testcase-detail/6439610474692608 https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=56804

(cherry picked from commit b602982)

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
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