msm: adsprpc: Fix array index underflow problem

Add check to restrict index underflow.This is to avoid
that it does not access invalid index.

Change-Id: Ib971033c5820ca4dab38ace3b106c7b1b42529e4
Acked-by: Gururaj Chalger <gchalger@qti.qualcomm.com>
Signed-off-by: Mohammed Nayeem Ur Rahman <mohara@codeaurora.org>