Memory leaks when parsing ARC headers

[KIC-8462852]

Hi,
There are two potential memory leaks when parsing ARC-Authentication-Results and ARC-Seal header fields.
This patch fixes these memory leaks.
Thx.
opendmarc.patch.txt

[thegushi]

A fix for this is available on the branch fix/issues-241-242-arc-parsing in https://github.com/thegushi/OpenDMARC — the two leaked allocations in opendmarc.c now have free() calls before the continue on parse failure, and unit tests covering the error paths have been added in libopendmarc/tests/test_arcares.c. This branch also addresses #242. Testing welcome.

[thegushi]

A fix for this has been submitted in PR #296 on the fix/arcseal-crashes-183-222-236 branch, which also covers the related crashes in #183, #186, #222, #236, #241, and #242.

The fix addresses two distinct crash modes:

1. SIGABRT (malformed headers with no = in a token, e.g. ARC-Seal: i=1; none): added NULL guards before strip_whitespace() is called in all three ARC parse functions.
2. SIGSEGV (RSA 3072-bit and larger key signatures producing b= values ≥512 bytes): removed the artificial 512-byte token cap from strip_whitespace() in both the arcseal and arcares parsers. Struct field sizes have also been bumped from 512 → 2048 bytes to accommodate large-key signatures without truncation.

If you are able to test against the fix/arcseal-crashes-183-222-236 branch and confirm whether it resolves your issue, that would be very helpful for getting this merged. Thank you!