VT: Check track's validity when dequeue access unit

[Problem] RTPSource crashes when it stops.
[Cause] AccessUnitComplete can happen even after Disconnect is happened.
        At that time, RTPSource try to access non-valid track
        that already cleared from track vector.
[Solution] Check the vector size before access a track.

Bug: 383206566
Flag: EXEMPT refactor
Test: Basic Video call function(b/383206566 #comment62)

Change-Id: I2833fe77df9f3ddaf695894fc87a1a80eb29b55f
Signed-off-by: Kim Sungyeon <sy85.kim@samsung.com>