Handle error case when no instances returned

Handle the error case that can arise when requests to run new AWS instances returns no errors but an empty struct of results.

Author: elliotrushton

drivers/amazonec2/amazonec2.go

@@ -664,7 +664,10 @@ func (d *Driver) innerCreate() error {
 
 		spotInstanceRequest, err := d.getClient().RequestSpotInstances(&req)
 		if err != nil {
-			return fmt.Errorf("Error request spot instance: %s", err)
+			return fmt.Errorf("Error request spot instance: %v", err)
+		}
+		if spotInstanceRequest == nil || len((*spotInstanceRequest).SpotInstanceRequests) < 1 {
+			return fmt.Errorf("Error requesting spot instance: %v", errors.New("Unexpected AWS API response"))
 		}
 		d.spotInstanceRequestId = *spotInstanceRequest.SpotInstanceRequests[0].SpotInstanceRequestId
 
@@ -740,7 +743,10 @@ func (d *Driver) innerCreate() error {
 		})
 
 		if err != nil {
-			return fmt.Errorf("Error launching instance: %s", err)
+			return fmt.Errorf("Error launching instance: %v", err)
+		}
+		if inst == nil || len(inst.Instances) < 1 {
+			return fmt.Errorf("Error launching instance: %v", errors.New("Unexpected AWS API response"))
 		}
 		instance = inst.Instances[0]
 	}