avcodec/aacdec: Dont fail if channels arent known yet

Fixes Ticket3312
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/aacdec.c

@@ -196,6 +196,9 @@ static int frame_configure_elements(AVCodecContext *avctx)
 
     /* get output buffer */
     av_frame_unref(ac->frame);
+    if (!avctx->channels)
+        return 1;
+
     ac->frame->nb_samples = 2048;
     if ((ret = ff_get_buffer(avctx, ac->frame, 0)) < 0)
         return ret;