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cgiutil: fix handling of malformed percent-encoding (#7511)

* cgiutil: fix handling of malformed percent-encoding

unescape_url() implicitly assumed that two hex digits
follow the percent sign without any checks.
This lead to an out-of-bounds read on malformed
percent-encoded URLs, such as "/?foo=bar%", and
undefined behavior if non-hex digits were supplied.

Fix this by verifying that two hex digits follow
the percent sign and only unescape it in this case.
In the malformed case, leave the percent-sign and
the following digits as-is.

Signed-off-by: Stefan Gloor <code@stefan-gloor.ch>

* Apply suggestions from code review

Co-authored-by: Even Rouault <even.rouault@spatialys.com>

* Formatting fix

* Apply suggestions from code review

Co-authored-by: Even Rouault <even.rouault@spatialys.com>

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Signed-off-by: Stefan Gloor <code@stefan-gloor.ch>
Co-authored-by: Even Rouault <even.rouault@spatialys.com>

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