[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().

pngrtran.c

@@ -1839,6 +1839,9 @@ png_read_transform_info(png_structrp png_ptr, png_inforp info_ptr)
 
          info_ptr->bit_depth = 8;
          info_ptr->num_trans = 0;
+
+         if (png_ptr->palette == NULL)
+            png_error (png_ptr, "Palette is NULL in indexed image");
       }
       else
       {