aac: check the maximum number of channels

Broken bitstreams could report a larger than specified number of
channels and cause outbound writes.

CC:libav-stable@libav.org

libavcodec/aacdec.c

@@ -141,6 +141,8 @@ 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))))