Fix status command if there's a standby instance in the ASG (#205)

* Fix status command if there's a standby instance in the ASG

* Review feedback

lib/moonshot/stack_asg_printer.rb

@@ -111,13 +111,21 @@ def create_instance_table(asg_info)
     end
 
     def instance_row(asg_instance, ec2_instance)
+      if ec2_instance
+        ip_address = ec2_instance.public_ip_address || "#{ec2_instance.private_ip_address} (PRV)"
+        uptime = uptime_format(ec2_instance.launch_time)
+      else
+        # We've seen race conditions where ASG tells us about instances that EC2 is no longer
+        # aware of.
+        ip_address = 'unknown'
+        uptime = 'unknown'
+      end
       [
         asg_instance.instance_id,
-        # @todo What about ASGs with only private IPs?
-        ec2_instance.public_ip_address,
+        ip_address,
         lifecycle_color(asg_instance.lifecycle_state),
         health_color(asg_instance.health_status),
-        uptime_format(ec2_instance.launch_time)
+        uptime
       ]
     end