`mb_encode_numericentity()` and `mb_decode_numericentity()` accepted
arbitrary zvals as `$convmap`, but ignored anything else than arrays.
This appears to be an unresolved relict of their ZPP conversion for
PHP 5.3[1]. We now expect an array in the first place.
We also expect `count($convmap)` to be a multiple of four (else we
throw a `ValueError`), and do no longer special case empty `$convmap`.
[1] <http://git.php.net/?p=php-src.git;a=commit;h=1c77f594294aee9d60e7309279c616c01c39ba9d>
