stack/avdt: Properly initialize AvdtpScb::curr_stream

curr_stream was not iniitalized and not reset in AvdtpScb::Reset,
causing potential use of uninitialized memory.

Bug: 296093782
Fix: 296093782
Flag: EXEMPT, minor bugfix
Test: m com.android.btservices
Change-Id: I9be460ea79f513dee1e00958a01a6f32590375a4