lavc/aac_ac3_parser: fix potential overflow when averaging bitrate

The new code is analog to how it's done in our mpegaudio parser.
Acked-by: Jun Zhao <barryjzhao@tencent.com>
Signed-off-by: Alexander Strasser <eclipse7@gmx.net>

libavcodec/aac_ac3_parser.c

@@ -100,9 +100,8 @@ get_next:
         /* Calculate the average bit rate */
         s->frame_number++;
         if (avctx->codec_id != AV_CODEC_ID_EAC3) {
-            avctx->bit_rate =
-                (s->last_bit_rate * (s->frame_number -1) + s->bit_rate)/s->frame_number;
-            s->last_bit_rate = avctx->bit_rate;
+            avctx->bit_rate +=
+                (s->bit_rate - avctx->bit_rate) / s->frame_number;
         }
     }
 

libavcodec/aac_ac3_parser.h

@@ -56,7 +56,6 @@ typedef struct AACAC3ParseContext {
 
     int need_next_header;
     int frame_number;
-    int last_bit_rate;
     enum AVCodecID codec_id;
 } AACAC3ParseContext;
 

tests/ref/fate/adtstoasc_ticket3715

-3e63cbb6bb6ec756d79fab2632fef305 *tests/data/fate/adtstoasc_ticket3715.mov
+29ef0632a8eb5c336bf45a1d5076626e *tests/data/fate/adtstoasc_ticket3715.mov
 33324 tests/data/fate/adtstoasc_ticket3715.mov
 #extradata 0:        2, 0x00340022
 #tb 0: 1/44100