Fix crash with conversations with "uknown contact" header.

We weren't accounting for ConversationAdapter header position when
getting the next and previous record in a conversation.

src/org/thoughtcrime/securesms/ConversationAdapter.java

@@ -190,10 +190,12 @@ public class ConversationAdapter <V extends View & BindableConversationItem>
   }
 
   @Override
-  protected void onBindItemViewHolder(ViewHolder viewHolder, @NonNull MessageRecord messageRecord, int position) {
-    long          start          = System.currentTimeMillis();
-    MessageRecord previousRecord = position < getItemCount() - 1 ? getRecordForPositionOrThrow(position + 1) : null;
-    MessageRecord nextRecord     = position > 0 ? getRecordForPositionOrThrow(position - 1) : null;
+  protected void onBindItemViewHolder(ViewHolder viewHolder, @NonNull MessageRecord messageRecord, int adjustedPosition) {
+
+    long          start            = System.currentTimeMillis();
+    int           rawPosition      = getRawCursorPosition(adjustedPosition);
+    MessageRecord previousRecord   = rawPosition < getItemCount() - 1 && !isFooterPosition(rawPosition + 1) ? getRecordForPositionOrThrow(rawPosition + 1) : null;
+    MessageRecord nextRecord       = rawPosition > 0 && !isHeaderPosition(rawPosition - 1) ? getRecordForPositionOrThrow(rawPosition - 1) : null;
 
     viewHolder.getView().bind(messageRecord,
                               Optional.fromNullable(previousRecord),

src/org/thoughtcrime/securesms/database/CursorRecyclerViewAdapter.java

@@ -211,6 +211,13 @@ public abstract class CursorRecyclerViewAdapter<VH extends RecyclerView.ViewHold
     return position - getFastAccessSize();
   }
 
+  protected int getRawCursorPosition(int position) {
+    if (hasHeaderView()) {
+      position += 1;
+    }
+    return position;
+  }
+
   protected int getFastAccessItemViewType(int position) {
     return 0;
   }