document null Project reference bug

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@494970 13f79535-47bb-0310-9956-ffa450edef68

WHATSNEW

@@ -31,6 +31,9 @@ Fixed bugs:
 * <apt> forks properly and so memory settings are picked up.
   Bug report 41280.
 
+* Regression: NPE was thrown when using <pathconvert> against a
+  (third-party instantiated) fileset with null Project reference.
+
 Other changes:
 --------------
 
@@ -3756,4 +3759,4 @@ Fixed bugs:
 
 * The packagelistloc attribute of <javadoc>'s <link> child will be
   resolved as a file (i.e. it is either absolute or relative to
-  basedir).
+  basedir).