Fix an off-by-one error in `png_do_check_palette_indexes`

The last byte of each row was ignored in a function that was executed
under the build flags `PNG_READ_CHECK_FOR_INVALID_INDEX_SUPPORTED` and
`PNG_WRITE_CHECK_FOR_INVALID_INDEX_SUPPORTED`.

This is a revert of a change previously applied in libpng-1.6.33beta01.
See SourceForge bug #269 at https://sourceforge.net/p/libpng/bugs/269/

Reviewed-by: John Bowler <jbowler@acm.org>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>