Fix binding LOB values in mysqli driver prepared statement

Running the DBAL test suite with mysqli driver I get a memory exhaustion in `BlobTest`. This is caused by the `MysqliStatement` trying to bind a LOB value before having stored the result set. According to [this comment](http://php.net/manual/en/mysqli-stmt.bind-result.php#57564) in the PHP docs this is necessary when dealing with LOBs. At least this solves the problem for me.