filter_parser: fix memory leak (#8614)

[tomas-sve]

Summary

This fixes a memory leak in the Parser Filter plugin.

If you have several keys with the same name in a map and use the parser filter plugin with reserve_data set to true then it can assign out_buf multiple times resulting in leaked memory. This fix makes it continue copying original fields if reserve_data is set but stop parsing them, similar to when reserve_data is set to false.

Fixes #8614

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

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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.

[tomas-sve]

Example config/data to reproduce the bug can be found in #8614

Valgrind output (valgrind --leak-check=full) after fix:

==2520027==
==2520027== HEAP SUMMARY:
==2520027== in use at exit: 0 bytes in 0 blocks
==2520027== total heap usage: 32,333 allocs, 32,333 frees, 12,107,683 bytes allocated
==2520027==
==2520027== All heap blocks were freed -- no leaks are possible
==2520027==
==2520027== For lists of detected and suppressed errors, rerun with: -s
==2520027== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Valgrind output before fix can also be found in #8614