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/postgre/postgre_driver.php b/system/database/drivers/postgre/postgre_driver.php
index 028dc19..40f21b1 100644
--- a/system/database/drivers/postgre/postgre_driver.php
+++ b/system/database/drivers/postgre/postgre_driver.php
@@ -204,8 +204,12 @@
 		{
 			return $this->data_cache['version'];
 		}
+		elseif ( ! $this->conn_id)
+		{
+			$this->initialize();
+		}
 
-		if (($pg_version = pg_version($this->conn_id)) === FALSE)
+		if ( ! $this->conn_id OR ($pg_version = pg_version($this->conn_id)) === FALSE)
 		{
 			return FALSE;
 		}