Unhandled case where call to AWS RunInstances returns empty list of reservations and no error results in panic

[elliotrushton]

In some rare cases calls to RunInstances returns an empty struct and no error https://github.com/docker/machine/blob/a555e4f7a8f518a8b1b174824c377e46cbfc4fe2/drivers/amazonec2/amazonec2.go#L723. In this case the later call that indexes into the result https://github.com/docker/machine/blob/a555e4f7a8f518a8b1b174824c377e46cbfc4fe2/drivers/amazonec2/amazonec2.go#L745 causes docker-machine to panic with
Error creating machine: Error in driver during machine creation: Panic in the driver: runtime error: index out of range

Deploy Kit handles this case by raising an error about an unexpected result from the AWS SDK https://github.com/docker/deploykit/blob/master/pkg/provider/aws/plugin/instance/ec2_instance.go#L272

[elliotrushton]

I have made a simple pull request that fixes this and prevents docker machine from panicking and crashing in this case: #4747