enforce NonNull on incoming MMS content location

Fixes #2959
Closes #2975

// FREEBIE

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

@@ -77,14 +77,18 @@ public class MmsDownloadJob extends MasterSecretJob {
       return;
     }
 
-    database.markDownloadState(messageId, MmsDatabase.Status.DOWNLOAD_CONNECTING);
+    try {
+      if (notification.get().getContentLocation() == null) {
+        throw new MmsException("Notification content location was null.");
+      }
 
-    String contentLocation = new String(notification.get().getContentLocation());
-    byte[] transactionId   = notification.get().getTransactionId();
+      database.markDownloadState(messageId, MmsDatabase.Status.DOWNLOAD_CONNECTING);
 
-    Log.w(TAG, "Downloading mms at " + Uri.parse(contentLocation).getHost());
+      String contentLocation = new String(notification.get().getContentLocation());
+      byte[] transactionId   = notification.get().getTransactionId();
+
+      Log.w(TAG, "Downloading mms at " + Uri.parse(contentLocation).getHost());
 
-    try {
       RetrieveConf retrieveConf = getMmsConnection(context).retrieve(contentLocation, transactionId);
       if (retrieveConf == null) {
         throw new MmsException("RetrieveConf was null");

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.NonNull;
 import android.support.annotation.Nullable;
 import android.util.Log;
 
@@ -58,7 +59,10 @@ public class IncomingLegacyMmsConnection extends LegacyMmsConnection implements
   }
 
   @Override
-  public @Nullable RetrieveConf retrieve(String contentLocation, byte[] transactionId) throws MmsRadioException, ApnUnavailableException, IOException {
+  public @Nullable RetrieveConf retrieve(@NonNull 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.NonNull;
 import android.support.annotation.Nullable;
 import android.telephony.SmsManager;
 import android.util.Log;
@@ -56,7 +57,9 @@ public class IncomingLollipopMmsConnection extends LollipopMmsConnection impleme
 
   @Override
   @TargetApi(VERSION_CODES.LOLLIPOP)
-  public synchronized @Nullable RetrieveConf retrieve(String contentLocation, byte[] transactionId) throws MmsException {
+  public synchronized @Nullable RetrieveConf retrieve(@NonNull String contentLocation,
+                                                      byte[] transactionId) throws MmsException
+  {
     beginTransaction();
 
     try {

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

@@ -1,5 +1,6 @@
 package org.thoughtcrime.securesms.mms;
 
+import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 
 import java.io.IOException;
@@ -8,5 +9,5 @@ import ws.com.google.android.mms.MmsException;
 import ws.com.google.android.mms.pdu.RetrieveConf;
 
 public interface IncomingMmsConnection {
-  @Nullable RetrieveConf retrieve(String contentLocation, byte[] transactionId) throws MmsException, MmsRadioException, ApnUnavailableException, IOException;
+  @Nullable RetrieveConf retrieve(@NonNull String contentLocation, byte[] transactionId) throws MmsException, MmsRadioException, ApnUnavailableException, IOException;
 }