This amends commit 4dc20ff6.

Starting with MariaDB 10.2, InnoDB defines
typedef size_t ulint;

The standard format for size_t uses the z modifier, for example,
"%zu" as in the macro ULINTPF.

"%lu" is wrong for size_t, because sizeof(unsigned long) can be
something else than sizeof(size_t). On Windows, the former would
always be 4 bytes, while size_t would be 4 or 8 bytes.