Better handle remove interface from router for unknown port uuid

When doing "Remove interface from router" and only port uuid is
provided; the lookup in hash map may return null. If so, there
needs to be a check against NPE.

Change-Id: Ibaea49ac3fdfc08e92c95e818e5ec82628822f3f
Ref: http://docs.openstack.org/api/openstack-network/2.0/content/PUT_removeRouterInterface__v2.0_routers_remove_router_interface_router_ext.html
Signed-off-by: Flavio Fernandes <ffernand@redhat.com>
Merge "Better handle remove interface from router for unknown port uuid"