avcodec/shorten: Fix discard of ‘const’ qualifier

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/shorten.c

@@ -563,7 +563,6 @@ static int shorten_decode_frame(AVCodecContext *avctx, AVFrame *frame,
     buf               = &s->bitstream[s->bitstream_index];
     buf_size         += s->bitstream_size;
     s->bitstream_size = buf_size;
-    memset(buf + buf_size, 0, AV_INPUT_BUFFER_PADDING_SIZE);
 
     /* do not decode until buffer has at least max_framesize bytes or
      * the end of the file has been reached */
@@ -583,10 +582,9 @@ static int shorten_decode_frame(AVCodecContext *avctx, AVFrame *frame,
             return ret;
 
         if (avpkt->size) {
-            int max_framesize;
+            int max_framesize = s->blocksize * s->channels * 8;
             void *tmp_ptr;
 
-            max_framesize = FFMAX(s->max_framesize, s->blocksize * s->channels * 8);
             tmp_ptr = av_fast_realloc(s->bitstream, &s->allocated_bitstream_size,
                                       max_framesize + AV_INPUT_BUFFER_PADDING_SIZE);
             if (!tmp_ptr) {
@@ -594,7 +592,10 @@ static int shorten_decode_frame(AVCodecContext *avctx, AVFrame *frame,
                 return AVERROR(ENOMEM);
             }
             s->bitstream = tmp_ptr;
-            s->max_framesize = max_framesize;
+            if (max_framesize > s->max_framesize)
+                memset(s->bitstream + s->max_framesize, 0, (max_framesize - s->max_framesize) +
+                                                            AV_INPUT_BUFFER_PADDING_SIZE);
+            s->max_framesize = FFMAX(s->max_framesize, max_framesize);
             *got_frame_ptr = 0;
             goto finish_frame;
         }