The URL percent-encoding decode function in libcurl is called
`curl_easy_unescape`. Internally, even if this function would be made to
allocate a unscape destination buffer larger than 2GB, it would return that
new length in a signed 32 bit integer variable, thus the length would get
either just truncated or both truncated and turned negative. That could then
lead to libcurl writing outside of its heap based buffer.

This can be triggered by a user on a 64bit system if the user can send in a
custom (very large) URL to a libcurl using program.

External References:

https://curl.haxx.se/docs/adv_20161102H.html