can not delete an instance when resizing

Description
===========

When an instance task state is RESIZE_MIGRATED or RESIZE_FINISH, delete the instance will hint an error,
I found some problems with the code, because my flavor contains vram and instance at a special task state, a TypeError or an AttributeError will throw out.

Delete when instance at resize_migrated task state, AttributeError will throw out.
Delete when instance at resize_finish task state, TypeError will throw out.

Steps to reproduce
==================
1.create an instance use flavor1,names server1
2.resize it but not resize confirm.
3.delete the instance quickly.
4.you can run " nova resize service1 flavor2 && sleep 2.0 && nova delete server1 ".

Just delete the instance quickly

When resize an instance, its task state will go through None => resize_migrating => resize_migrated => resize_finish => None, the purpose of sleep is to catch the target state，resize_migrated or resize_finish.

Environment
===========

RDO Newton
DevStack (master=pike)

Logs & Configs
==============

 Traceback (most recent call last):
   File "/usr/lib/python2.7/site-packages/nova/api/openstack/extensions.py", line 338, in wrapped
     return f(*args, **kwargs)
   File "/usr/lib/python2.7/site-packages/nova/api/openstack/compute/servers.py", line 914, in delete
     self._delete(req.environ['nova.context'], req, id)
   File "/usr/lib/python2.7/site-packages/nova/api/openstack/compute/servers.py", line 768, in _delete
     self.compute_api.delete(context, instance)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 166, in inner
     return function(self, context, instance, *args, **kwargs)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 173, in _wrapped
     return fn(self, context, instance, *args, **kwargs)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 147, in inner
     return f(self, context, instance, *args, **kw)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 2048, in delete
     self._delete_instance(context, instance)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 2039, in _delete_instance
     task_state=task_states.DELETING)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 1815, in _delete
     quotas.rollback()
   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
     self.force_reraise()
   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
     six.reraise(self.type_, self.value, self.tb)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 1735, in _delete
     project_id, user_id)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 1878, in _create_reservations
     instance_memory_mb = old_flavor.memory_mb + vram_mb
 TypeError: unsupported operand type(s) for +: 'int' and 'unicode'

OR

Traceback (most recent call last):
   File "/usr/lib/python2.7/site-packages/nova/api/openstack/extensions.py", line 338, in wrapped
     return f(*args, **kwargs)
   File "/usr/lib/python2.7/site-packages/nova/api/openstack/compute/servers.py", line 914, in delete
     self._delete(req.environ['nova.context'], req, id)
   File "/usr/lib/python2.7/site-packages/nova/api/openstack/compute/servers.py", line 768, in _delete
     self.compute_api.delete(context, instance)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 166, in inner
     return function(self, context, instance, *args, **kwargs)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 173, in _wrapped
     return fn(self, context, instance, *args, **kwargs)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 147, in inner
     return f(self, context, instance, *args, **kw)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 2048, in delete
     self._delete_instance(context, instance)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 2039, in _delete_instance
     task_state=task_states.DELETING)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 1815, in _delete
     quotas.rollback()
   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
     self.force_reraise()
   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
     six.reraise(self.type_, self.value, self.tb)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 1735, in _delete
     project_id, user_id)
   File "/usr/lib/python2.7/site-packages/nova/compute/api.py", line 1876, in _create_reservations
     instance_vcpus = old_flavor.vcpus
 AttributeError: 'NoneType' object has no attribute 'vcpus'