floss: A2DP: Avoid use-after-free of UIPC

If the pointer is not null, then an UIPC could be in the background and
accesses the structure, which causes use-after-free.

Bug: 373228946
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: bluetooth_AdapterAUHealth.all_floss
Flag: EXEMPT, Floss-only change
Change-Id: I3f420b623b83830481d952012f6067410e9c6ba1