[libpng16] Deleted a now-redundant #define png_error from pngvalid.c
[libpng16] Check for info_ptr == NULL early in png_read_end() so we don't need

to run all the png_handle_*() and depend on them to return if info_ptr == NULL.
This improves the performance of png_read_end(png_ptr, NULL) and makes
it more robust against future programming errors.