Fix using mbuf after returning it to mbuf free list for redis mset.

[huachaohuang]

[andyqzb]

I merge the bugfix in #486 , but using mbuf_length on a mbuf is not a good way after putting the mbuf to the free list. So can you create a new pr based on the master, I want to merge your code.

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[TysonAndre]

I merge the bugfix in 486 , but using mbuf_length on a mbuf is not a good way after putting the mbuf to the free list. So can you create a new pr based on the master, I want to merge your code.

My understanding of this: twemproxy is single threaded so it doesn't cause issues in practice, but would agree in principle (e.g. if mbuf_free were to be replaced with an actuall free call as a debug compile option to debug memory leaks or use-after free)