sstables: fix lz4 compress
sstables: fix lz4 compress

compress_lz4 must account the input length prepended to the output
buffer.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@cloudius-systems.com>
Reviewed-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@cloudius-systems.com>

raphaelsc/scylla,aruanruan/scylla,scylladb/scylla,duarten/scylla,senseb/scylla,rentongzhang/scylla,wildinto/scylla,raphaelsc/scylla,victorbriz/scylla,bowlofstew/scylla,phonkee/scylla,capturePointer/scylla,respu/scylla,tempbottle/scylla,rentongzhang/scylla,linearregression/scylla,gwicke/scylla,glommer/scylla,respu/scylla,kangkot/scylla,scylladb/scylla,bowlofstew/scylla,acbellini/scylla,victorbriz/scylla,kangkot/scylla,justintung/scylla,stamhe/scylla,guiquanz/scylla,shaunstanislaus/scylla,tempbottle/scylla,duarten/scylla,acbellini/scylla,capturePointer/scylla,linearregression/scylla,tempbottle/scylla,shaunstanislaus/scylla,glommer/scylla,glommer/scylla,guiquanz/scylla,rluta/scylla,aruanruan/scylla,eklitzke/scylla,rluta/scylla,bowlofstew/scylla,linearregression/scylla,senseb/scylla,dwdm/scylla,kjniemi/scylla,shaunstanislaus/scylla,justintung/scylla,kjniemi/scylla,duarten/scylla,respu/scylla,phonkee/scylla,stamhe/scylla,acbellini/scylla,wildinto/scylla,kjniemi/scylla,stamhe/scylla,phonkee/scylla,guiquanz/scylla,wildinto/scylla,rentongzhang/scylla,avikivity/scylla,eklitzke/scylla,rluta/scylla,senseb/scylla,scylladb/scylla,scylladb/scylla,asias/scylla,raphaelsc/scylla,capturePointer/scylla,asias/scylla,asias/scylla,dwdm/scylla,justintung/scylla,dwdm/scylla,kangkot/scylla,gwicke/scylla,eklitzke/scylla,aruanruan/scylla,victorbriz/scylla,gwicke/scylla,avikivity/scylla,avikivity/scylla