Curl_dedotdotify(): always nul terminate returned string.

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