ec2 inventory: Verify that a security group exists before trying to use its name as a key in inventory.

plugins/inventory/ec2.py

@@ -384,6 +384,7 @@ class Ec2Inventory(object):
         
         # Inventory: Group by security group
         try:
+            if instance.security_group:
                 key = self.to_safe("security_group_" + instance.security_group.name)
                 self.push(self.inventory, key, dest)
         except AttributeError: