Superframe index: Parse correct number of frame sizes

If there are N frames in a superframe, the superframe
index will enumerate the sizes of only the first
(N - 1) frames. The code assumed that all N frame
sizes are coded.

The size of the final frame in the superframe is
calculated as follows:
(data_size - index_size - sum_of_N-1_frame_sizes)

Change-Id: Ia53ae27455a4f024eec311bf7356a00e8e0d449a

av1/decoder/decoder.c

diff --git a/av1/decoder/decoder.c b/av1/decoder/decoder.c
index 6c85d75..e75ecc6 100644
--- a/av1/decoder/decoder.c
+++ b/av1/decoder/decoder.c
@@ -553,10 +553,10 @@
       const uint8_t *x = &data[data_sz - index_sz + 1];
 
       // Frames has a maximum of 8 and mag has a maximum of 4.
-      uint8_t clear_buffer[32];
-      assert(sizeof(clear_buffer) >= frames * mag);
+      uint8_t clear_buffer[28];
+      assert(sizeof(clear_buffer) >= (frames - 1) * mag);
       if (decrypt_cb) {
-        decrypt_cb(decrypt_state, x, clear_buffer, frames * mag);
+        decrypt_cb(decrypt_state, x, clear_buffer, (frames - 1) * mag);
         x = clear_buffer;
       }