curl: URL unescape heap overflow via integer truncation

The URL percent-encoding decode function in libcurl before 7.51.0 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.