config_format: cf_yaml: Plug memory leaks on exception for processing variants on plugin elements

[cosmo0920]

These memory leaks are detected by OSSFuzz.
(struct parser_state *)state->variant_kvlist_key and (struct parser_state *)state->variant should be destroyed before tweardowning parser state value is cleaned up.

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Testing
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• 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

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• Run local packaging test showing all targets (including any new ones) build.
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Backporting

• Backport to latest stable release.

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