Show verify button when we have a device to verify

rather than when we don't

src/components/views/settings/KeyBackupPanel.js

@@ -234,7 +234,7 @@ export default class KeyBackupPanel extends React.PureComponent {
                 }
 
                 let verifyButton;
-                if (!sig.device || !sig.device.isVerified()) {
+                if (sig.device && !sig.device.isVerified()) {
                     verifyButton = <div><br /><AccessibleButton className="mx_UserSettings_button"
                             onClick={this._verifyDevice} data-sigindex={i}>
                         { _t("Verify...") }