Prevent unwanted line breaks in <pre> tags

By default, Mantis wraps text within <pre> tags at 100 chars (see
$g_wrap_in_preformatted_text option).

If the text within the tag contains Unicode extended characters, the
regex performing the wrap does not count them properly, as pcre works on
bytes, causing line breaks to be added in the wrong place.

Adding 'u' modifier to the regex fixes the behavior, as suggested by
cproensa.

Fixes #21410

core/string_api.php

@@ -123,7 +123,8 @@ function string_nl2br( $p_string, $p_wrap = 100 ) {
 				#     if other encoded characters are a problem
 				$t_piece = preg_replace( '/ /', ' ', $t_piece );
 				if( ON == config_get( 'wrap_in_preformatted_text' ) ) {
-					$t_output .= preg_replace( '/([^\n]{' . $p_wrap . ',}?[\s]+)(?!<\/pre>)/', "$1\n", $t_piece );
+					# Use PCRE_UTF8 modifier to ensure a correct char count
+					$t_output .= preg_replace( '/([^\n]{' . $p_wrap . ',}?[\s]+)(?!<\/pre>)/u', "$1", $t_piece );
 				} else {
 					$t_output .= $t_piece;
 				}