pack-gelf: fix OSS-Fuzz issue 5076752961110016

[DavidKorczynski]

Signed-off-by: davkor david@adalogics.com

This solves an issue in parsing an msgpack-c object related to gelf data. The main problem is that a wrong variable is used to get the msgpack data, and this results in various bugs, amongst other things a heap overflow in flb_msgpack_gelf_value_ext (OSS-Fuzz issue 5076752961110016)

@edsiper please verify this one, i.e. that we need to use v instead of o. v is the variable used all other places for getting the val and val_len variables.

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[edsiper]

thanks