Revert "Fix Bug #79448 0 is a valid Unicode codepoint, but mb_substitute_character(0) fails"

This commit brings some substantial changes in behaviour due to the weird implementation.
This will be fixed in master due to BC concerns.

This reverts commit 1333b46.

Author: Girgias

ext/mbstring/mbstring.c

@@ -2038,7 +2038,7 @@ PHP_FUNCTION(mb_detect_order)
 
 static inline int php_mb_check_code_point(zend_long cp)
 {
-	if (cp < 0 || cp >= 0x110000) {
+	if (cp <= 0 || cp >= 0x110000) {
 		/* Out of Unicode range */
 		return 0;
 	}