This triggers lAPrev (e_a[0]) on the first SBR frame to be -1. The spec is
somewhat ambiguous to what this value should be but this increases the accuracy
of al_sbr_e_44_1 and similar streams from 14 bits to 15 bits.


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22408 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b