Address: Handle getHostname when mAddress is null

k9mail-library/src/main/java/com/fsck/k9/mail/Address.java

@@ -78,6 +78,10 @@ public class Address implements Serializable {
     }
 
     public String getHostname() {
+        if (mAddress == null) {
+            return null;
+        }
+
         int hostIdx = mAddress.lastIndexOf("@");
 
         if (hostIdx == -1) {

k9mail-library/src/test/java/com/fsck/k9/mail/AddressTest.java

@@ -148,4 +148,13 @@ public class AddressTest {
 
         assertFalse(result);
     }
+
+    @Test
+    public void getHostname_withoutAddress_isNull() throws Exception {
+        Address address = Address.parse("Alice")[0];
+
+        String result = address.getHostname();
+
+        assertNull(result);
+    }
 }