Description:
------------
The MySQLi mysqli_ssl_set() function is broken because 
it doesn't convert empty strings into NULLs, which is 
what the MySQL C client library function mysql_ssl_set() 
wants.

This patch fixes this bug:

http://www.trachtenberg.com/patches/mysql_ssl_set.txt

Reproduce code:
---------------
$db = mysqli_init();
mysqli_ssl_set($db, 'client-key.pem', 'client-cert.pem', 'cacert.pem', NULL, NULL);

mysqli_real_connect($db, 'localhost', 'foo', 'bar', 'baz');

Expected result:
----------------
Sucessful connection.

Actual result:
--------------
PHP Warning:  mysqli_real_connect(): SSL connection 
error