tftp: fix memory leak on too long filename

Fixes

$ valgrind --leak-check=full ~/install-curl-git/bin/curl tftp://localhost/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaz

==9752== Memcheck, a memory error detector
==9752== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==9752== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==9752== Command: /home/even/install-curl-git/bin/curl tftp://localhost/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaz
==9752==
curl: (71) TFTP file name too long

==9752==
==9752== HEAP SUMMARY:
==9752== 505 bytes in 1 blocks are definitely lost in loss record 11 of 11
==9752==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==9752==    by 0x4E61CED: Curl_urldecode (in /home/even/install-curl-git/lib/libcurl.so.4.4.0)
==9752==    by 0x4E75868: tftp_state_machine (in /home/even/install-curl-git/lib/libcurl.so.4.4.0)
==9752==    by 0x4E761B6: tftp_do (in /home/even/install-curl-git/lib/libcurl.so.4.4.0)
==9752==    by 0x4E711B6: multi_runsingle (in /home/even/install-curl-git/lib/libcurl.so.4.4.0)
==9752==    by 0x4E71D00: curl_multi_perform (in /home/even/install-curl-git/lib/libcurl.so.4.4.0)
==9752==    by 0x4E6950D: curl_easy_perform (in /home/even/install-curl-git/lib/libcurl.so.4.4.0)
==9752==    by 0x40E0B7: operate_do (in /home/even/install-curl-git/bin/curl)
==9752==    by 0x40E849: operate (in /home/even/install-curl-git/bin/curl)
==9752==    by 0x402693: main (in /home/even/install-curl-git/bin/curl)

Fixes https://oss-fuzz.com/v2/testcase-detail/5232311106797568
Credit to OSS Fuzz

Closes #1808