Fix an OOB access bug in BtaAvCo::GetNextSourceDataPacket

If the length of the packet is less than 4 or the offset is 0
OOB access is triggered.

Bug: 259939364
Test: manual
Ignore-AOSP-First: security
Merged-In: I11a3ebf20c45e9e69a4008a7d7271470e6235fe1
Change-Id: I11a3ebf20c45e9e69a4008a7d7271470e6235fe1