XFA - Prevent breaking errors in Binder, when searchNode doesn't return data (issue 13756)

As can be seen in the code (see below), the `searchNode` helper function will return `null` in some cases and all of its call-sites should protect against that before attempting to access the returned data.
While only one of these changes were necessary to fix the breaking errors in issue 13756, in order to prevent future bugs I've added similar defensive code throughout this file.

 - 07955fa1d3/src/core/xfa/som.js (L169)
 - 07955fa1d3/src/core/xfa/som.js (L239)
 - 07955fa1d3/src/core/xfa/som.js (L254)

src/core/xfa/bind.js

@@ -198,34 +198,36 @@ class Binder {
         continue;
       }
 
-      const [node] = searchNode(
+      const nodes = searchNode(
         this.root,
         dataNode,
         ref,
         false /* = dotDotAllowed */,
         false /* = useCache */
       );
-      if (!node) {
+      if (!nodes) {
         warn(`XFA - Invalid reference: ${ref}.`);
         continue;
       }
+      const [node] = nodes;
 
       if (!node[$isDescendent](this.data)) {
         warn(`XFA - Invalid node: must be a data node.`);
         continue;
       }
 
-      const [targetNode] = searchNode(
+      const targetNodes = searchNode(
         this.root,
         formNode,
         target,
         false /* = dotDotAllowed */,
         false /* = useCache */
       );
-      if (!targetNode) {
+      if (!targetNodes) {
         warn(`XFA - Invalid target: ${target}.`);
         continue;
       }
+      const [targetNode] = targetNodes;
 
       if (!targetNode[$isDescendent](formNode)) {
         warn(`XFA - Invalid target: must be a property or subproperty.`);
@@ -337,34 +339,36 @@ class Binder {
           continue;
         }
 
-        const [labelNode] = searchNode(
+        const labelNodes = searchNode(
           this.root,
           node,
           labelRef,
           true /* = dotDotAllowed */,
           false /* = useCache */
         );
-        if (!labelNode) {
+        if (!labelNodes) {
           warn(`XFA - Invalid label: ${labelRef}.`);
           continue;
         }
+        const [labelNode] = labelNodes;
 
         if (!labelNode[$isDescendent](this.datasets)) {
           warn(`XFA - Invalid label: must be a datasets child.`);
           continue;
         }
 
-        const [valueNode] = searchNode(
+        const valueNodes = searchNode(
           this.root,
           node,
           valueRef,
           true /* = dotDotAllowed */,
           false /* = useCache */
         );
-        if (!valueNode) {
+        if (!valueNodes) {
           warn(`XFA - Invalid value: ${valueRef}.`);
           continue;
         }
+        const [valueNode] = valueNodes;
 
         if (!valueNode[$isDescendent](this.datasets)) {
           warn(`XFA - Invalid value: must be a datasets child.`);

test/pdfs/issue13756.pdf.link

@@ -0,0 +1 @@
+https://github.com/mozilla/pdf.js/files/6842290/Attestation_d.occupation.pdf

test/test_manifest.json

@@ -1347,6 +1347,14 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "issue13756",
+       "file": "pdfs/issue13756.pdf",
+       "md5": "67040c6df3b5039fcbc81bf63b7c7f98",
+       "link": true,
+       "rounds": 1,
+       "enableXfa": true,
+       "type": "eq"
+    },
     {  "id": "issue9262",
        "file": "pdfs/issue9262_reduced.pdf",
        "md5": "5347ce2d7b3866625c22e115fd90e0de",