http: avoid length underflow in Curl_compareheader - #22338
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Since curl 8.20.0, the comma scan in
Curl_compareheadersubtracts the bytes consumed in each round from the remaining value length, but skipping trailing blanks can move the parsing point past the trimmed value end, wrapping the unsigned length. The scan then continues beyond the value and can match a token after an embedded newline. This adds a guard that stops the loop at the value end, with new unit 1625 cases covering both shapes.