be safer when processing parts in AttachmentDownloadJob

Closes #4166
// FREEBIE

test/unitTest/java/org/thoughtcrime/securesms/crypto/MasterCipherTest.java

@@ -0,0 +1,24 @@
+package org.thoughtcrime.securesms.crypto;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.powermock.core.classloader.annotations.PowerMockIgnore;
+import org.thoughtcrime.securesms.BaseUnitTest;
+import org.whispersystems.libaxolotl.InvalidMessageException;
+
+@PowerMockIgnore("javax.crypto.*")
+public class MasterCipherTest extends BaseUnitTest {
+  private MasterCipher masterCipher;
+
+  @Before
+  @Override
+  public void setUp() throws Exception {
+    super.setUp();
+    masterCipher = new MasterCipher(masterSecret);
+  }
+
+  @Test(expected = InvalidMessageException.class)
+  public void testEncryptBytesWithZeroBody() throws Exception {
+    masterCipher.decryptBytes(new byte[]{});
+  }
+}