Don't break when loading an Excel file containing a chartsheet

Test case found here: http://www.excel-easy.com/examples/excel-files/chart-sheet.xlsx

lib/xlsx/xform/book/workbook-xform.js

@@ -158,9 +158,16 @@ utils.inherits(WorkbookXform, BaseXform, {
         return;
       }
       worksheet = model.worksheetHash['xl/' + rel.Target];
-      worksheet.name = sheet.name;
-      worksheet.id = sheet.id;
-      worksheets[index++] = worksheet;
+      // If there are "chartsheets" in the file, rel.Target will
+      // come out as chartsheets/sheet1.xml or similar here, and
+      // that won't be in model.worksheetHash.
+      // As we don't have the infrastructure to support chartsheets,
+      // we will ignore them for now:
+      if (worksheet) {
+        worksheet.name = sheet.name;
+        worksheet.id = sheet.id;
+        worksheets[index++] = worksheet;
+      }
     });
 
     // reconcile print areas

spec/integration/worksheet.spec.js

@@ -630,4 +630,12 @@ describe('Worksheet', function() {
         });
     });
   });
+
+  it('Should not break when importing an Excel file that contains a chartsheet', function() {
+    return new Excel.Workbook().xlsx.readFile(path.resolve(__dirname, 'data', 'chart-sheet.xlsx'))
+      .then(function(workbook) {
+        expect(workbook).to.have.property('worksheets');
+        expect(workbook.worksheets).to.have.length(1);
+      });
+  });
 });