Prevent NPE when thumbnail click not set

Fixes #3042
Fixes #3044
Closes #3046
// FREEBIE

src/org/thoughtcrime/securesms/components/ThumbnailView.java

@@ -177,7 +177,11 @@ public class ThumbnailView extends ForegroundImageView {
 
     @Override
     public void onClick(View view) {
+      if (listener != null) {
         listener.onClick(view, slide);
+      } else {
+        Log.w(TAG, "onClick, but no thumbnail click listener attached.");
+      }
     }
   }