Bug 57925: Add unit test and a simple fix to at least avoid the NPE when Workbooks have invalid external references

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1703999 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/poi/ss/formula/OperationEvaluationContext.java

@@ -94,7 +94,11 @@ public final class OperationEvaluationContext {
 		if (externalSheet == null || externalSheet.getWorkbookName() == null) {
 			// sheet is in same workbook
 			targetEvaluator = _bookEvaluator;
-			otherFirstSheetIndex = _workbook.getSheetIndex(externalSheet.getSheetName());
+			if(externalSheet == null) {
+			    otherFirstSheetIndex = 0;
+			} else {
+			    otherFirstSheetIndex = _workbook.getSheetIndex(externalSheet.getSheetName());
+			}
 			
 			if (externalSheet instanceof ExternalSheetRange) {
 			    String lastSheetName = ((ExternalSheetRange)externalSheet).getLastSheetName();

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

@@ -2846,4 +2846,22 @@ public final class TestBugs extends BaseTestBugzillaIssues {
 
         wb.close();
     }
+    
+    @Test
+    public void test57925() throws IOException {
+        Workbook wb = HSSFTestDataSamples.openSampleWorkbook("57925.xls");
+        
+        wb.getCreationHelper().createFormulaEvaluator().evaluateAll();
+        
+        for(int i = 0;i < wb.getNumberOfSheets();i++) {
+            Sheet sheet = wb.getSheetAt(i);
+            for(Row row : sheet) {
+                for(Cell cell : row) {
+                    new DataFormatter().formatCellValue(cell);
+                }
+            }
+        }
+        
+        wb.close();
+    }
 }