in_splunk: ensure releasing tag_from_record

[nokute78]

tag_from_record is allocated at tag_key.
https://github.com/fluent/fluent-bit/blob/v2.1.10/plugins/in_splunk/splunk_prot.c#L202

It will be only released upon success at process_flb_log_append
https://github.com/fluent/fluent-bit/blob/v2.1.10/plugins/in_splunk/splunk_prot.c#L296

This patch is to ensure releasing tag_from_record.

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