assign a default value to machine_map 



git-svn-id: http://test.kernel.org/svn/autotest/trunk@851 592f7852-d20e-0410-864c-8624ca9c26a4
diff --git a/tko/db.py b/tko/db.py
index cf5c46c..7b1f265 100644
--- a/tko/db.py
+++ b/tko/db.py
@@ -46,6 +46,8 @@
 		machine_map = os.path.join(dir, 'machines')
 		if os.path.exists(machine_map):
 			self.machine_map = machine_map
+		else:
+			self.machine_map = None
 		self.machine_group = {}