Check for invalid packet size in the smacker demuxer.

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit e055932f56)

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/smacker.c

@@ -292,6 +292,8 @@ static int smacker_read_packet(AVFormatContext *s, AVPacket *pkt)
             }
             flags >>= 1;
         }
+        if (frame_size < 0)
+            return AVERROR_INVALIDDATA;
         if (av_new_packet(pkt, frame_size + 768))
             return AVERROR(ENOMEM);
         if(smk->frm_size[smk->cur_frame] & 1)