aac: Wait to know the channels before allocating frame

The channel configuration can be delivered only by the PCE,
try to parse it first and not try to decode until a channel
configuration is set.

CC: libav-stable@libav.org

libavcodec/aacdec.c

@@ -2821,8 +2821,9 @@ static int aac_decode_frame_int(AVCodecContext *avctx, void *data,
         }
     }
 
-    if ((err = frame_configure_elements(avctx)) < 0)
-        goto fail;
+    if (avctx->channels)
+        if ((err = frame_configure_elements(avctx)) < 0)
+            goto fail;
 
     // The FF_PROFILE_AAC_* defines are all object_type - 1
     // This may lead to an undefined profile being signaled
@@ -2833,6 +2834,9 @@ static int aac_decode_frame_int(AVCodecContext *avctx, void *data,
     while ((elem_type = get_bits(gb, 3)) != TYPE_END) {
         elem_id = get_bits(gb, 4);
 
+        if (!avctx->channels && elem_type != TYPE_PCE)
+            goto fail;
+
         if (elem_type < TYPE_DSE) {
             if (!(che=get_che(ac, elem_type, elem_id))) {
                 av_log(ac->avctx, AV_LOG_ERROR, "channel element %d.%d is not allocated\n",
@@ -2919,6 +2923,11 @@ static int aac_decode_frame_int(AVCodecContext *avctx, void *data,
         }
     }
 
+    if (!avctx->channels) {
+        *got_frame_ptr = 0;
+        return 0;
+    }
+
     spectral_to_sample(ac);
 
     multiplier = (ac->oc[1].m4ac.sbr == 1) ? ac->oc[1].m4ac.ext_sample_rate > ac->oc[1].m4ac.sample_rate : 0;