return error if loop has ended before decoding image

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@8503 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/gifdec.c

@@ -273,6 +273,7 @@ static int gif_parse_next_image(GifState *s)
             return -1;
         }
     }
+    return -1;
 }
 
 static int gif_decode_init(AVCodecContext *avctx)