Guard against adding children to ended structure elements.

lib/structure_element.js

@@ -48,6 +48,10 @@ class PDFStructureElement {
   }
 
   add(child) {
+    if (this._ended) {
+      throw new Error(`Cannot add child to already-ended structure element`);
+    }
+
     if (!this.dictionary.data.K) {
       this.dictionary.data.K = [];
     }

tests/unit/markings.spec.js

@@ -412,6 +412,20 @@ EMC
         `endobj`
       ]);
     });
+
+    test('validation', () => {
+      let struct;
+
+      struct = document.struct('Foo').end();
+      expect(() => {
+        struct.add(document.struct('Bar'));
+      }).toThrow();
+
+      struct = document.struct('Foo', []);
+      expect(() => {
+        struct.add(document.struct('Bar'));
+      }).toThrow();
+    });
   });
 
   describe('accessible document', () => {