This fixes potential out-of-buffer access on "file:./" URL
$ valgrind curl "file:./"
==24516== Memcheck, a memory error detector
==24516== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==24516== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==24516== Command: /home/even/install-curl-git/bin/curl file:./
==24516==
==24516== Conditional jump or move depends on uninitialised value(s)
==24516== at 0x4C31F9C: strcmp (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==24516== by 0x4EBB315: seturl (urlapi.c:801)
==24516== by 0x4EBB568: parseurl (urlapi.c:861)
==24516== by 0x4EBC509: curl_url_set (urlapi.c:1199)
==24516== by 0x4E644C6: parseurlandfillconn (url.c:2044)
==24516== by 0x4E67AEF: create_conn (url.c:3613)
==24516== by 0x4E68A4F: Curl_connect (url.c:4119)
==24516== by 0x4E7F0A4: multi_runsingle (multi.c:1440)
==24516== by 0x4E808E5: curl_multi_perform (multi.c:2173)
==24516== by 0x4E7558C: easy_transfer (easy.c:686)
==24516== by 0x4E75801: easy_perform (easy.c:779)
==24516== by 0x4E75868: curl_easy_perform (easy.c:798)
Was originally spotted by
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10637
Credit to OSS-Fuzz
Closes#3039
In order for this API to fully work for libcurl itself, it now offers a
CURLU_GUESS_SCHEME flag that makes it "guess" scheme based on the host
name prefix just like libcurl always did. If there's no known prefix, it
will guess "http://".
Separately, it relaxes the check of the host name so that IDN host names
can be passed in as well.
Both these changes are necessary for libcurl itself to use this API.
Assisted-by: Daniel Gustafsson
Closes#3018