flicvideo: fix crash on flic files with invalid frame size

Add a check in flic_decode_frame_8BPP(), in case chunk_size is >
frame_size issue a warning and resize chunk_size to frame_size, in
order to avoid out-of-buffer reads.

Fix roundup issue #2520, trac issue #69.
Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>

libavcodec/flicvideo.c

@@ -181,6 +181,11 @@ static int flic_decode_frame_8BPP(AVCodecContext *avctx,
     /* iterate through the chunks */
     while ((frame_size > 0) && (num_chunks > 0)) {
         chunk_size = AV_RL32(&buf[stream_ptr]);
+        if (chunk_size > frame_size) {
+            av_log(avctx, AV_LOG_WARNING,
+                   "Invalid chunk_size = %u > frame_size = %u\n", chunk_size, frame_size);
+            chunk_size = frame_size;
+        }
         stream_ptr += 4;
         chunk_type = AV_RL16(&buf[stream_ptr]);
         stream_ptr += 2;