msm: camera: reqmgr: check validity of last_applied_idx

last_applied_idx is updated on every successful applied
request and the buf_done. After buf_done last_applied_idx
is set to -1. While accessing last applied req the same
index is used, if the last_applied_idx is -1 accessing
last applied request will result in slab out of bound
error, so check last_applied_idx before accessing
last applied request.

CRs-Fixed: 2840329
Change-Id: I8f49b9df097859cde20e651149167db7316976bb
Signed-off-by: Tejas Prajapati <tpraja@codeaurora.org>