Avoid an NPE that says nothing (ex. could happen if the message template is wrong).

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/branches/Temp_ProcessingFeedback@634712 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/fop/util/text/AdvancedMessageFormat.java

@@ -273,7 +273,7 @@ public class AdvancedMessageFormat {
                 }
             }
             if (!handled) {
-                target.append(obj.toString());
+                target.append(String.valueOf(obj));
             }
         }
     }