in_systemd: fix memory leak

[bpetermannS11]

Fix a memory leak in the systemd input plugin.
If option "lowercase" was on, the temporary buffer to convert a key to the lower-case string was not freed.

Fixes #9769

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Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [N/A] Run local packaging test showing all targets (including any new ones) build.
• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [N/A] Documentation required for this feature

Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[bpetermannS11]

Example configuration

[INPUT]
    name              systemd
    tag               journald
    db                /var/log/fluent-bit/fluent-bit-journal.db
    lowercase         on
    path              /var/log/journal

[bpetermannS11]

Valgrind/debug output

valgrind --leak-check=full ~bpetermann/fluent-bit-bp/build/bin/fluent-bit -c fluent-bit.conf
==71649== Memcheck, a memory error detector
==71649== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==71649== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==71649== Command: /home/bpetermann/fluent-bit-bp/build/bin/fluent-bit -c fluent-bit.conf
==71649==
Fluent Bit v3.2.4
* Copyright (C) 2015-2024 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

______ _                  _    ______ _ _           _____  _____
|  ___| |                | |   | ___ (_) |         |____ |/ __  \
| |_  | |_   _  ___ _ __ | |_  | |_/ /_| |_  __   __   / /`' / /'
|  _| | | | | |/ _ \ '_ \| __| | ___ \ | __| \ \ / /   \ \  / /
| |   | | |_| |  __/ | | | |_  | |_/ / | |_   \ V /.___/ /./ /___
\_|   |_|\__,_|\___|_| |_|\__| \____/|_|\__|   \_/ \____(_)_____/


[2024/12/27 13:00:29] [ info] [fluent bit] version=3.2.4, commit=d77c06dc6c, pid=71649
[2024/12/27 13:00:29] [ info] [storage] ver=1.5.2, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2024/12/27 13:00:29] [ info] [simd    ] disabled
[2024/12/27 13:00:29] [ info] [cmetrics] version=0.9.9
[2024/12/27 13:00:29] [ info] [ctraces ] version=0.5.7
[2024/12/27 13:00:29] [ info] [input:systemd:systemd.0] initializing
[2024/12/27 13:00:29] [ info] [input:systemd:systemd.0] storage_strategy='memory' (memory only)
[2024/12/27 13:00:30] [ info] [input:systemd:systemd.0] seek_cursor=s=e33aefeeefa44645b4ee468c56cf6de8;i=999... OK
[2024/12/27 13:00:30] [ info] [sp] stream processor started
^C[2024/12/27 13:00:45] [engine] caught signal (SIGINT)
[2024/12/27 13:00:45] [ warn] [engine] service will shutdown in max 5 seconds
[2024/12/27 13:00:45] [ info] [input] pausing systemd.0
[2024/12/27 13:00:45] [ info] [engine] service has stopped (0 pending tasks)
[2024/12/27 13:00:45] [ info] [input] pausing systemd.0
==71649==
==71649== HEAP SUMMARY:
==71649==     in use at exit: 0 bytes in 0 blocks
==71649==   total heap usage: 2,944 allocs, 2,944 frees, 1,425,156 bytes allocated
==71649==
==71649== All heap blocks were freed -- no leaks are possible
==71649==
==71649== For lists of detected and suppressed errors, rerun with: -s
==71649== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

[cosmo0920]

@edsiper This could be better than #9764. So, we need to review on this.