aacdec: Fix out of array writes (stack).

Set the element to channel vector (e2c_vec) size to be the maximum
number of aac channel elements. This makes it slightly larger than it
needs to be because CCEs are never mapped to output channel locations.

Also add a check that all input tags (legal or not) will fit.

Split from FFmpeg commit a8d67efa53

Signed-off-by: Alex Converse <alex.converse@gmail.com>

libavcodec/aacdec.c

@@ -223,10 +223,13 @@ static int count_paired_channels(uint8_t (*layout_map)[3], int tags, int pos, in
 static uint64_t sniff_channel_order(uint8_t (*layout_map)[3], int tags)
 {
     int i, n, total_non_cc_elements;
-    struct elem_to_channel e2c_vec[MAX_ELEM_ID] = {{ 0 }};
+    struct elem_to_channel e2c_vec[4*MAX_ELEM_ID] = {{ 0 }};
     int num_front_channels, num_side_channels, num_back_channels;
     uint64_t layout;
 
+    if (FF_ARRAY_ELEMS(e2c_vec) < tags)
+        return 0;
+
     i = 0;
     num_front_channels =
         count_paired_channels(layout_map, tags, AAC_CHANNEL_FRONT, &i);