Bluetooth-OPP: Check URI of sending files before entry cleanup

Checking the URI of sending files before removing the file entry
from Hashmap. Otherwise, during BT ON/OFF scenario there is a
possibility of URI becoming NULL that will lead to BT app crash.

CRs-Fixed: 721360
Change-Id: Ic7e8966d38e30948228e14a740cd3051b345da31