avdtp: Fix possible unbalance ref count

avdtp: Fix possible unbalance ref count

It has been reported that under some unknown circuntances the reference
number of AVDTP sessions can go bellow 0 (loop around) causing a crash:

bluetoothd[5633]: profiles/audio/avdtp.c:avdtp_ref() 0x56a882b8a500: ref=-2101995743
bluetoothd[5633]: profiles/audio/a2dp.c:discover_cb() err (nil)
WARNING crash_reporter[10707]: [user] Received crash notification for bluetoothd[5633]

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>