[libpng17] Handle zero-length PLTE chunk or NULL palette with png_error()

instead of png_chunk_report(), which by default issues a warning
rather than an error, leading to later reading from a NULL pointer
(png_ptr->palette) in png_do_expand_palette().

pngset.c

@@ -528,7 +528,7 @@ png_set_PLTE(png_structrp png_ptr, png_inforp info_ptr,
 #        endif
       ))
    {
-      png_chunk_report(png_ptr, "Invalid palette", PNG_CHUNK_ERROR);
+      png_error(png_ptr, "Invalid palette");
       return;
    }