Improve Atrac id generating and fix crash.

It may fix #2155 and some other crash.
The crash may happen as following:
    
    AtracID = 0; bmg playing 
    AtracID = 1; voice; release;
    AtracID = 2; voice; release;
    ....
    AtracID = 255; voice; release;
    AtracID = 0; crash ( already exists)
And a valid atracID in a real PSP seems only to be 0 ~ 6 from uofw.