fix crash after MemorizingTrustManager's TOFU/POP Always/Once/Abort prompt

If there is an unknown HTTPS certificate, MemorizingTrustManager puts up a
prompt to ask whether the user wants to trust the certificate.  It comes at
a weird time in the lifecycle of the dialogs, so the previous dialog might
be null.  Therefore add a null check.

This situation should probably be improved and better integrated.

src/org/fdroid/fdroid/UpdateService.java

@@ -180,7 +180,7 @@ public class UpdateService extends IntentService implements ProgressListener {
 
             if (finished) {
                 forwardEvent(EVENT_FINISHED);
-                if (dialog.isShowing()) {
+                if (dialog != null && dialog.isShowing()) {
                     try {
                         dialog.dismiss();
                     } catch (IllegalArgumentException e) {