aacdec: avoid an out-of-bounds write

Also move the check in the case it is actually used.

CC: libav-stable@libav.org
Bug-Id: CID 1087090

libavcodec/aacdec.c

@@ -143,8 +143,6 @@ static av_cold int che_configure(AACContext *ac,
                                  enum ChannelPosition che_pos,
                                  int type, int id, int *channels)
 {
-    if (*channels >= MAX_CHANNELS)
-        return AVERROR_INVALIDDATA;
     if (che_pos) {
         if (!ac->che[type][id]) {
             if (!(ac->che[type][id] = av_mallocz(sizeof(ChannelElement))))
@@ -152,6 +150,8 @@ static av_cold int che_configure(AACContext *ac,
             ff_aac_sbr_ctx_init(ac, &ac->che[type][id]->sbr);
         }
         if (type != TYPE_CCE) {
+            if (*channels >= MAX_CHANNELS - 2)
+                return AVERROR_INVALIDDATA;
             ac->output_element[(*channels)++] = &ac->che[type][id]->ch[0];
             if (type == TYPE_CPE ||
                 (type == TYPE_SCE && ac->oc[1].m4ac.ps == 1)) {