Fixed issue where a zero length libre office document with a water mark ...

src/ooxml/java/org/apache/poi/xwpf/usermodel/XWPFDocument.java

@@ -92,7 +92,7 @@ Licensed to the Apache Software Foundation (ASF) under one or more
     /**
      * Keeps track on all id-values used in this document and included parts, like headers, footers, etc.
      */
-    private IdentifierManager drawingIdManager = new IdentifierManager(1L,4294967295L);
+    private IdentifierManager drawingIdManager = new IdentifierManager(0L,4294967295L);
     protected List<XWPFFooter> footers = new ArrayList<XWPFFooter>();
     protected List<XWPFHeader> headers = new ArrayList<XWPFHeader>();
     protected List<XWPFComment> comments = new ArrayList<XWPFComment>();

src/ooxml/testcases/org/apache/poi/xwpf/usermodel/TestXWPFDocument.java

@@ -337,6 +337,21 @@ public void testPictureHandlingComplex() throws IOException, InvalidFormatExcept
 	    
 	    doc.getPackage().revert();
 	}
+    public void testZeroLengthLibreOfficeDocumentWithWaterMarkHeader() throws IOException {
+        XWPFDocument doc = XWPFTestDataSamples.openSampleDocument("zero-length.docx");
+        POIXMLProperties properties = doc.getProperties();
+
+        assertNotNull(properties.getCoreProperties());
+
+        XWPFHeader headerArray = doc.getHeaderArray(0);
+        assertEquals(1, headerArray.getAllPictures().size());
+        assertEquals("image1.png", headerArray.pictures.get(0).getFileName());
+        assertEquals("", headerArray.getText());
+
+        POIXMLProperties.ExtendedProperties extendedProperties = properties.getExtendedProperties();
+        assertNotNull(extendedProperties);
+        assertEquals(0, extendedProperties.getUnderlyingProperties().getCharacters());
+    }
 
     public void testSettings(){
         XWPFSettings settings = new XWPFSettings();