Fix XSS in adm_config_report.php

Yelin and Zhangdongsheng from VenusTech http://www.venustech.com.cn/
reported a vulnerability in the Configuration Report page, allowing an
attacker to inject arbitrary code through a crafted 'config_option'
parameter.

Sanitize the parameter prior to output, to ensure HTML special
characters are properly escaped.

Ported from 1.3.x commit c9e5b1d.

Fixes #22579

adm_config_report.php

@@ -580,7 +580,7 @@ function check_config_value( $p_config ) {
 					<input type="text" name="config_option" class="input-sm"
 						   value="<?php echo string_display_line( $t_edit_option ); ?>"
 						   size="64" maxlength="64" />
-					<input type="hidden" name="original_config_option" value="<?php echo $t_edit_option; ?>" />
+					<input type="hidden" name="original_config_option" value="<?php echo string_display_line( $t_edit_option ); ?>" />
 				</td>
 			</tr>