Fixed #28

 o Checking zOut for null otherwise i get a NPE
   during test with maven-archiver.

Author: khmarbaise

src/main/java/org/codehaus/plexus/archiver/zip/AbstractZipArchiver.java

@@ -779,7 +779,9 @@ protected void close()
         {
             if ( zipArchiveOutputStream != null )
             {
-                zOut.writeTo( zipArchiveOutputStream );
+                if ( zOut != null) {
+                  zOut.writeTo( zipArchiveOutputStream );
+                }
                 zipArchiveOutputStream.close();
             }
         }