runCommand: Fix unnecessary nil check

[abhinav]

On the relevant line, wait is guaranteed to be non-nil
because there's a nil check a few lines ago that always returns.

wait, runErr := impl.Run(s, cmd.args...)
if wait == nil {
	if async && runErr == nil {
		return cmdError(cmd, errors.New("internal error: async command returned a nil WaitFunc"))
	}
	return checkStatus(cmd, runErr)
}