podman umount: error out if called with no args

Inspired by containers#2684, I wrote a CI test to look for other such
instances in which a command is invoked without a required
argument. 'podman umount' seems to be the only one.

Signed-off-by: Ed Santiago <santiago@redhat.com>

cmd/podman/umount.go

@@ -60,6 +60,10 @@ func umountCmd(c *cliconfig.UmountValues) error {
 	force := c.Force
 	umountAll := c.All
 
+	if len(c.PodmanCommand.InputArgs) < 1 && !umountAll {
+		return errors.Errorf("you must provide at least one mounted container, or the -a flag")
+	}
+
 	containers, err := getAllOrLatestContainers(&c.PodmanCommand, runtime, -1, "all")
 	if err != nil {
 		if len(containers) == 0 {