msm: camera: isp: Check whether flush list is empty

We will get invalid req from flush_list if the
flush list is empty when we get each req by
list API list_for_each_entry_safe. This change
checks whether flush list is empty before
accessing this list.

CRs-Fixed: 2885145
Change-Id: I9b2828b03706a0f7412828905cc0a9e28d15fd76
Signed-off-by: Depeng Shao <depengs@codeaurora.org>