Don't try to call module method if module did not load

Change-Id: I95f7f7fce326aaaf5f2559055c0dc6a72d2559f1
diff --git a/_grains/keystone_policy.py b/_grains/keystone_policy.py
index 1812c03..2155b12 100644
--- a/_grains/keystone_policy.py
+++ b/_grains/keystone_policy.py
@@ -7,8 +7,9 @@
     path = "/etc/keystone/policy.json"
     __opts__ = salt.config.minion_config('/etc/salt/minion')
     keystone_policy_mod = salt.loader.raw_mod(__opts__, 'keystone_policy', None)
-    result = keystone_policy_mod['keystone_policy.rule_list'](path)
-    if result and 'Error' not in result:
-        return {'keystone_policy': result}
+    if keystone_policy_mod:
+        result = keystone_policy_mod['keystone_policy.rule_list'](path)
+        if result and 'Error' not in result:
+            return {'keystone_policy': result}
     return {}