lcl: don't overwrite input memory.

If the PNG filter is enabled, a PNG-style filter will run over the
input buffer, writing into the buffer. Therefore, if no zlib compression
was used, ensure that we copy into a temporary buffer, otherwise we
overwrite user-provided input data.

libavcodec/lcldec.c

@@ -236,9 +236,14 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size, AVPac
          * gives a file with ZLIB fourcc, but frame is really uncompressed.
          * To be sure that's true check also frame size */
         if (c->compression == COMP_ZLIB_NORMAL && c->imgtype == IMGTYPE_RGB24 &&
-            len == width * height * 3)
+            len == width * height * 3) {
+            if (c->flags & FLAG_PNGFILTER) {
+                memcpy(c->decomp_buf, encoded, len);
+                encoded = c->decomp_buf;
+            } else {
                 break;
-        if (c->flags & FLAG_MULTITHREAD) {
+            }
+        } else if (c->flags & FLAG_MULTITHREAD) {
             int ret;
             mthread_inlen = AV_RL32(encoded);
             mthread_inlen = FFMIN(mthread_inlen, len - 8);