LU-8165 target: detect race by checking last_rcvd slot index

LU-8165 target: detect race by checking last_rcvd slot index

A race can occur on Server during Client connection and
concurent eviction, when Client's last_rcvd slot index has still
not been assigned (-1).
This patch adds a check to address such condition.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: Ifead82719a0dc9411f1b79d6c8c59eb9ef339fa5
Reviewed-on: http://review.whamcloud.com/20328
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Grégoire Pichon <gregoire.pichon@bull.net>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>