fix a condition not seen until a recent expansion of the stress test. Gracefully ignore missing/invalid external sheet references in one more path (there were several already with comments like "this seems to be what Excel does in this case")

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1856655 13f79535-47bb-0310-9956-ffa450edef68

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

@@ -179,7 +179,7 @@ public final class SheetNameFormatter {
 
 		int len = rawSheetName.length();
 		if(len < 1) {
-			throw new RuntimeException("Zero length string is an invalid sheet name");
+		    return false; // some cases we get missing external references, resulting in empty sheet names
 		}
 		if(Character.isDigit(rawSheetName.charAt(0))) {
 			// sheet name with digit in the first position always requires delimiting

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

@@ -17,9 +17,12 @@
 
 package org.apache.poi.hssf.usermodel;
 
+import org.apache.poi.hssf.HSSFTestDataSamples;
 import org.apache.poi.ss.formula.EvaluationSheet;
 import org.apache.poi.ss.usermodel.BaseTestXEvaluationSheet;
+import org.apache.poi.ss.usermodel.Name;
 import org.apache.poi.ss.usermodel.Sheet;
+import org.junit.Test;
 
 import java.util.AbstractMap;
 import java.util.Map;
@@ -30,4 +33,15 @@ public class TestHSSFEvaluationSheet extends BaseTestXEvaluationSheet {
         HSSFSheet sheet = new HSSFWorkbook().createSheet();
         return new AbstractMap.SimpleEntry<>(sheet, new HSSFEvaluationSheet(sheet));
     }
+    
+    @Test
+    public void testMissingExternalName() {
+        HSSFWorkbook wb = HSSFTestDataSamples.openSampleWorkbook("external_name.xls");
+        for (Name name : wb.getAllNames()) {
+            // this sometimes causes exceptions
+            if(!name.isFunctionName()) {
+                name.getRefersToFormula();
+            }
+        }
+    }
 }