avcodec/aacdec: Dont fail if channels arent known yet

Fixes Ticket3312

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 676a395ab9)

Conflicts:
	libavcodec/aacdec.c

libavcodec/aacdec.c

@@ -190,6 +190,9 @@ static int frame_configure_elements(AVCodecContext *avctx)
         }
     }
 
+    if (!avctx->channels)
+        return 1;
+
     /* get output buffer */
     ac->frame.nb_samples = 2048;
     if ((ret = ff_get_buffer(avctx, &ac->frame)) < 0) {