atrac3: set the getbits context the right buffer_end

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/atrac3.c

@@ -742,7 +742,7 @@ static int decode_frame(AVCodecContext *avctx, const uint8_t *databuf,
 
 
         /* set the bitstream reader at the start of the second Sound Unit*/
-        init_get_bits(&q->gb, ptr1, avctx->block_align * 8);
+        init_get_bits(&q->gb, ptr1, (avctx->block_align - i) * 8);
 
         /* Fill the Weighting coeffs delay buffer */
         memmove(q->weighting_delay, &q->weighting_delay[2],