Fix DB drivers version() implementations that don't execute a query

Fails if called prior to the DB connection initialization.
diff --git a/system/database/drivers/mysqli/mysqli_driver.php b/system/database/drivers/mysqli/mysqli_driver.php
index 7d2507b..ae17703 100644
--- a/system/database/drivers/mysqli/mysqli_driver.php
+++ b/system/database/drivers/mysqli/mysqli_driver.php
@@ -173,9 +173,16 @@
 	 */
 	public function version()
 	{
-		return isset($this->data_cache['version'])
-			? $this->data_cache['version']
-			: $this->data_cache['version'] = $this->conn_id->server_info;
+		if (isset($this->data_cache['version']))
+		{
+			return $this->data_cache['version'];
+		}
+		elseif ( ! $this->conn_id)
+		{
+			$this->initialize();
+		}
+
+		return $this->data_cache['version'] = $this->conn_id->server_info;
 	}
 
 	// --------------------------------------------------------------------