Description:
------------
The offset checking in mb_stripos and mb_strripos doesn't match the case sensitive equivalents with regard to character counts rather than byte counts. More importantly entering a negative offset in mb_strripos results in a "Offset not contained in string." message which was not expected.

Suggested code changes.

mb_stripos function add the check:
	if (offset < 0 || (unsigned long)offset > (unsigned long)mbfl_strlen(&old_haystack)) {
		php_error_docref(NULL TSRMLS_CC, E_WARNING, "Offset not contained in string.");
		RETURN_FALSE;
	}

mb_strripos function add the check: 
	if ((offset > 0 && offset > mbfl_strlen(&old_haystack)) ||
		(offset < 0 && -offset > mbfl_strlen(&old_haystack))) {
		php_error_docref(NULL TSRMLS_CC, E_NOTICE, "Offset is greater than the length of haystack string");
		RETURN_FALSE;
	}


php_mb_stripos function remove the check:
		if (offset < 0 || (unsigned long)offset > haystack.len) {
			php_error_docref(NULL TSRMLS_CC, E_WARNING, "Offset not contained in string.");
			break;
		}



Reproduce code:
---------------
<?php
var_dump(mb_strripos("abc abc abc", "b", -3));
?>

Expected result:
----------------
int(5)

Actual result:
--------------
Warning: mb_strripos(): Offset not contained in string. in C:\udata-eclipse\p8\a.phpcode\testmb.php on line 2
bool(false)