Fix the issue when docker exec a paused container, it will always hang.

Add the test case of this issue.

Docker-DCO-1.1-Signed-off-by: Chen Chao <cc272309126@gmail.com> (github: cc272309126)
Upstream-commit: 1bb02117db80e75f406f6c63d8d50680c1569019
Component: engine

components/engine/daemon/exec.go

@@ -98,7 +98,9 @@ func (d *Daemon) getActiveContainer(name string) (*Container, error) {
 	if !container.IsRunning() {
 		return nil, fmt.Errorf("Container %s is not running", name)
 	}
-
+	if container.IsPaused() {
+		return nil, fmt.Errorf("Container %s is paused, unpause the container before exec", name)
+	}
 	return container, nil
 }