Crash on screen rotate with OpenPGP handled message
### Expected behaviour

Message should be re-rendered in landscape.
### Actual behaviour

Application crashes with following exception.

```
03-29 08:40:55.041 12045-12045/com.fsck.k9.debug E/AndroidRuntime: FATAL EXCEPTION: main
                                                                   Process: com.fsck.k9.debug, PID: 12045
                                                                   java.util.NoSuchElementException
                                                                       at java.util.ArrayDeque.removeFirst(ArrayDeque.java:248)
                                                                       at com.fsck.k9.ui.crypto.MessageCryptoHelper.onCryptoFinished(MessageCryptoHelper.java:454)
                                                                       at com.fsck.k9.ui.crypto.MessageCryptoHelper.onCryptoSuccess(MessageCryptoHelper.java:438)
                                                                       at com.fsck.k9.ui.crypto.MessageCryptoHelper.handleCryptoOperationSuccess(MessageCryptoHelper.java:420)
                                                                       at com.fsck.k9.ui.crypto.MessageCryptoHelper.handleCryptoOperationResult(MessageCryptoHelper.java:379)
                                                                       at com.fsck.k9.ui.crypto.MessageCryptoHelper.onCryptoOperationReturned(MessageCryptoHelper.java:353)
                                                                       at com.fsck.k9.ui.crypto.MessageCryptoHelper.access$400(MessageCryptoHelper.java:49)
                                                                       at com.fsck.k9.ui.crypto.MessageCryptoHelper$4.onReturn(MessageCryptoHelper.java:250)
                                                                       at org.openintents.openpgp.util.OpenPgpApi$OpenPgpAsyncTask.onPostExecute(OpenPgpApi.java:298)
                                                                       at org.openintents.openpgp.util.OpenPgpApi$OpenPgpAsyncTask.onPostExecute(OpenPgpApi.java:279)
                                                                       at android.os.AsyncTask.finish(AsyncTask.java:651)
                                                                       at android.os.AsyncTask.-wrap1(AsyncTask.java)
                                                                       at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:668)
                                                                       at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                       at android.os.Looper.loop(Looper.java:148)
                                                                       at android.app.ActivityThread.main(ActivityThread.java:5422)
                                                                       at java.lang.reflect.Method.invoke(Native Method)
                                                                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
```

Email available on request.
### Steps to reproduce
1. Receive a message that has been signed
2. Open the message.
3. Rotate the phone to landscape
### Environment

K-9 Mail version: commit 74c6e76 (current master)
OpenKeychain version: 3.9.4
Android version: 6.0.1

Account type (IMAP, POP3, WebDAV/Exchange): IMAP

diff --git a/k9mail/src/main/java/com/fsck/k9/ui/messageview/MessageViewFragment.java b/k9mail/src/main/java/com/fsck/k9/ui/messageview/MessageViewFragment.java
index fccf71fd9..bd535fa89 100644
--- a/k9mail/src/main/java/com/fsck/k9/ui/messageview/MessageViewFragment.java
+++ b/k9mail/src/main/java/com/fsck/k9/ui/messageview/MessageViewFragment.java
@@ -734,6 +734,7 @@ public class MessageViewFragment extends Fragment implements ConfirmationDialogF
             } else {
                 onLoadMessageFromDatabaseFinished(message);
             }
+            getLoaderManager().destroyLoader(LOCAL_MESSAGE_LOADER_ID);
         }
 
         @Override
['k9mail/src/main/java/com/fsck/k9/ui/messageview/MessageViewFragment.java']