Only issue this warning if we actually have one non-null and one null value.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@5461 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/java/com/threerings/io/FieldMarshaller.java

@@ -75,7 +75,8 @@ public abstract class FieldMarshaller
             }
             if (reader != null && writer != null) {
                 return new MethodFieldMarshaller(reader, writer);
-            } else if (reader == null || writer == null) {
+            }
+            if ((reader == null && writer != null) || (writer == null && reader != null)) {
                 log.warning("Class contains one but not both custom field reader and writer",
                             "class", field.getDeclaringClass().getName(), "field", field.getName(),
                             "reader", reader, "writer", writer);