Fix NPE when RetrieveConf is null

Fixes #2828
Closes #2829
// FREEBIE

src/org/thoughtcrime/securesms/jobs/MmsDownloadJob.java

@@ -86,6 +86,9 @@ public class MmsDownloadJob extends MasterSecretJob {
 
     try {
       RetrieveConf retrieveConf = getMmsConnection(context).retrieve(contentLocation, transactionId);
+      if (retrieveConf == null) {
+        throw new MmsException("RetrieveConf was null");
+      }
       storeRetrievedMms(masterSecret, contentLocation, messageId, threadId, retrieveConf);
     } catch (ApnUnavailableException e) {
       Log.w(TAG, e);

src/org/thoughtcrime/securesms/mms/IncomingLegacyMmsConnection.java

@@ -18,6 +18,7 @@ package org.thoughtcrime.securesms.mms;
 
 import android.content.Context;
 import android.net.Uri;
+import android.support.annotation.Nullable;
 import android.util.Log;
 
 import org.apache.http.Header;
@@ -57,7 +58,7 @@ public class IncomingLegacyMmsConnection extends LegacyMmsConnection implements
   }
 
   @Override
-  public RetrieveConf retrieve(String contentLocation, byte[] transactionId) throws MmsRadioException, ApnUnavailableException, IOException {
+  public @Nullable RetrieveConf retrieve(String contentLocation, byte[] transactionId) throws MmsRadioException, ApnUnavailableException, IOException {
     MmsRadio radio = MmsRadio.getInstance(context);
     Apn contentApn = new Apn(contentLocation, apn.getProxy(), Integer.toString(apn.getPort()), apn.getUsername(), apn.getPassword());
     if (isCdmaDevice()) {

src/org/thoughtcrime/securesms/mms/IncomingLollipopMmsConnection.java

@@ -21,6 +21,7 @@ import android.content.Context;
 import android.content.Intent;
 import android.os.Build.VERSION;
 import android.os.Build.VERSION_CODES;
+import android.support.annotation.Nullable;
 import android.telephony.SmsManager;
 import android.util.Log;
 
@@ -55,7 +56,7 @@ public class IncomingLollipopMmsConnection extends LollipopMmsConnection impleme
 
   @Override
   @TargetApi(VERSION_CODES.LOLLIPOP)
-  public synchronized RetrieveConf retrieve(String contentLocation, byte[] transactionId) throws MmsException {
+  public synchronized @Nullable RetrieveConf retrieve(String contentLocation, byte[] transactionId) throws MmsException {
     beginTransaction();
 
     try {

src/org/thoughtcrime/securesms/mms/IncomingMmsConnection.java

@@ -1,10 +1,12 @@
 package org.thoughtcrime.securesms.mms;
 
+import android.support.annotation.Nullable;
+
 import java.io.IOException;
 
 import ws.com.google.android.mms.MmsException;
 import ws.com.google.android.mms.pdu.RetrieveConf;
 
 public interface IncomingMmsConnection {
-  RetrieveConf retrieve(String contentLocation, byte[] transactionId) throws MmsException, MmsRadioException, ApnUnavailableException, IOException;
+  @Nullable RetrieveConf retrieve(String contentLocation, byte[] transactionId) throws MmsException, MmsRadioException, ApnUnavailableException, IOException;
 }