Avoid IndexOutOfRangeException.

If there is no next message, it should return to a MessageList.

src/com/fsck/k9/activity/MessageList.java

@@ -1356,14 +1356,12 @@ public class MessageList extends K9FragmentActivity implements MessageListFragme
 
     @Override
     public void showNextMessageOrReturn() {
-        if (K9.messageViewReturnToList()) {
+        if (K9.messageViewReturnToList() || !showNextMessage()) {
             if (mDisplayMode == DisplayMode.SPLIT_VIEW) {
                 showMessageViewPlaceHolder();
             } else {
                 showMessageList();
             }
-        } else {
-            showNextMessage();
         }
     }
 
@@ -1377,18 +1375,24 @@ public class MessageList extends K9FragmentActivity implements MessageListFragme
         mActionBarSubject.setMessageHeader(header);
     }
 
-    private void showNextMessage() {
+    private boolean showNextMessage() {
         MessageReference ref = mMessageViewFragment.getMessageReference();
         if (ref != null) {
-            mMessageListFragment.openNext(ref);
+            if (mMessageListFragment.openNext(ref)) {
+                return true;
+            }
         }
+        return false;
     }
 
-    private void showPreviousMessage() {
+    private boolean showPreviousMessage() {
         MessageReference ref = mMessageViewFragment.getMessageReference();
         if (ref != null) {
-            mMessageListFragment.openPrevious(ref);
+            if (mMessageListFragment.openPrevious(ref)) {
+                return true;
+            }
         }
+        return false;
     }
 
     private void showMessageList() {

src/com/fsck/k9/fragment/MessageListFragment.java

@@ -2854,22 +2854,24 @@ public class MessageListFragment extends SherlockFragment implements OnItemClick
         }
     }
 
-    public void openPrevious(MessageReference messageReference) {
+    public boolean openPrevious(MessageReference messageReference) {
         int position = getPosition(messageReference);
         if (position <= 0) {
-            return;
+            return false;
         }
 
         openMessageAtPosition(position - 1);
+        return true;
     }
 
-    public void openNext(MessageReference messageReference) {
+    public boolean openNext(MessageReference messageReference) {
         int position = getPosition(messageReference);
         if (position < 0 || position == mAdapter.getCount() - 1) {
-            return;
+            return false;
         }
 
         openMessageAtPosition(position + 1);
+        return true;
     }
 
     public boolean isFirst(MessageReference messageReference) {