src:parse: plug memory leaks in nullable handling

[slyon]

Description

src:parse: plug memory leaks in nullable handling, adding some more tests to check for additional cases.

Follow-up for #299

Fixes:

$ LD_LIBRARY_PATH=_build/src/ valgrind --leak-check=full _build/tests/ctests/test_netplan_parser
[...]
==71129==
==71129== HEAP SUMMARY:
==71129==     in use at exit: 179,509 bytes in 2,284 blocks
==71129==   total heap usage: 8,205 allocs, 5,921 frees, 1,319,847 bytes allocated
==71129==
[...]
==71129==
==71129== 2,554 (1,536 direct, 1,018 indirect) bytes in 1 blocks are definitely lost in loss record 1,828 of 1,835
==71129==    at 0x4E050C5: malloc (vg_replace_malloc.c:393)
==71129==    by 0x49B92D1: yaml_parser_load (in /usr/lib/x86_64-linux-gnu/libyaml-0.so.2.0.9)
==71129==    by 0x11968A: load_yaml_from_fd (parse.c:88)
==71129==    by 0x12450E: netplan_parser_load_nullable_overrides (parse.c:3501)
==71129==    by 0x124A38: test_netplan_parser_load_nullable_overrides (test_netplan_parser.c:53)
==71129==    by 0x485E4E0: ??? (in /usr/lib/x86_64-linux-gnu/libcmocka.so.0.7.0)
==71129==    by 0x485EEC4: _cmocka_run_group_tests (in /usr/lib/x86_64-linux-gnu/libcmocka.so.0.7.0)
==71129==    by 0x1252DA: main (test_netplan_parser.c:228)
==71129==
==71129== LEAK SUMMARY:
==71129==    definitely lost: 1,536 bytes in 1 blocks
==71129==    indirectly lost: 1,018 bytes in 30 blocks
==71129==      possibly lost: 608 bytes in 2 blocks
==71129==    still reachable: 159,603 bytes in 2,071 blocks
==71129==         suppressed: 0 bytes in 0 blocks
==71129== Reachable blocks (those to which a pointer was found) are not shown.
==71129== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==71129==
==71129== For lists of detected and suppressed errors, rerun with: -s
==71129== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 0 from 0)
==71163== Memcheck, a memory error detector
==71163== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==71163== Using Valgrind-3.20.0 and LibVEX; rerun with -h for copyright info
==71163== Command: _build/tests/ctests/test_netplan_error
==71163==

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[daniloegea]

lgtm