fix images disappearing when in recyclerviews

Fixes #3142
Closes #3288
// FREEBIE

src/org/thoughtcrime/securesms/ConversationItem.java

@@ -166,6 +166,7 @@ public class ConversationItem extends LinearLayout {
   }
 
   public void unbind() {
+    mediaThumbnail.clear();
   }
 
   public MessageRecord getMessageRecord() {