Bug 57163: Fix possible NullPointerException when a spreadsheet has no LinkTable and we try to adjust it during sheet removal.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1673168 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/poi/hssf/model/InternalWorkbook.java

@@ -774,13 +774,14 @@ public final class InternalWorkbook {
             }
         }
         
+        if (linkTable != null) {
             // also tell the LinkTable about the removed sheet
             // +1 because we already removed it from the count of sheets!
             for(int i = sheetIndex+1;i < getNumSheets()+1;i++) {
-            // also update the link-table as otherwise references might point at invalid sheets
                 linkTable.removeSheet(i);
             }
         }
+    }
 
     /**
      * make the tabid record look like the current situation.

src/testcases/org/apache/poi/hssf/usermodel/TestBugs.java

@@ -2716,4 +2716,14 @@ public final class TestBugs extends BaseTestBugzillaIssues {
         Workbook wb = openSample("57456.xls");
         wb.close();
     }
+    
+    @Test
+    public void test57163() throws IOException {
+        Workbook wb = openSample("57163.xls");
+        
+        while (wb.getNumberOfSheets() > 1) {
+            wb.removeSheetAt(1);
+        }
+        wb.close();
+    }
 }