fix: memory leak in NvDsEventMsgMeta

[vieduy]

Hi Team,

I realize that when send event through msg broker, the memory is increase by the time. I found the problem at pyds.alloc_nvds_event_msg_meta(user_event_meta). The problem is we don't free srcData in event_msg_meta_release_func. Hope this will help anyone meet the problem about memory leak when send event by nvmsgbroker plugin.

Please review the changes and let me know if you have any questions or require further adjustments.