fix NPE when unbinding ConversationItem

fixes #3329
fixes #3320
// FREEBIE

src/org/thoughtcrime/securesms/ConversationItem.java

@@ -166,8 +166,10 @@ public class ConversationItem extends LinearLayout {
   }
 
   public void unbind() {
+    if (mediaThumbnail != null) {
       mediaThumbnail.clear();
     }
+  }
 
   public MessageRecord getMessageRecord() {
     return messageRecord;