Fix a bug for msg->mlen

[deep011]

Function msg_append() already added the dst->mlen.

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[TysonAndre]

This pr seems to be correct, function msg_append() already added the dst->mlen.

A message (dst) is made up of 1 or more mbufs, and the bytes is the length of the msg, which is the sum of the length of the mbufs. msg_append/mbuf_insert add individual mbufs from the source to the destination.

As for how twemproxy is currently working despite that, I expect it's because mlen isn't used and that the code that generates redis responses acts on the mbuf list - the length might only be used for logging

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