Fix: string type writer

kernel/write.c

@@ -25,7 +25,9 @@ void type_writer(zval *value, zend_long row, zend_long columns, xls_resource_wri
     zend_uchar value_type = Z_TYPE_P(value);
 
     if (value_type == IS_STRING) {
-        worksheet_write_string(res->worksheet, lxw_row, lxw_col, Z_STRVAL_P(value), format_handle);
+        zend_string *_zs_value = zval_get_string(value);
+        worksheet_write_string(res->worksheet, lxw_row, lxw_col, ZSTR_VAL(_zs_value), format_handle);
+        zend_string_release(_zs_value);
         return;
     }