Old PDF export fail with custom CSS rules

This is a regression caused by XWIKI-23376 which changed the color output to use CSS Color Level 4 hex-with-alpha notation. However, FOP 2.3 only knows #RGB/#RRGGBB. In PropertyTokenizer.nextColor, 4 hex digits give len % 3 != 0, so it falls back to scanRestOfName(), whose while (++exprIndex < exprLength) pushes the index one past the end — expr.substring(0, 6) on a 5-char string. Hence Range [0, 6) out of bounds for length 5, a RuntimeException that aborts the whole export instead of being a recoverable PropertyException.