Guard JS http-batch-store accessing deleted chunk (#3134)

Workaround for https://github.com/attic-labs/noms/issues/3133.

js/noms/package.json

@@ -1,7 +1,7 @@
 {
   "name": "@attic/noms",
   "license": "Apache-2.0",
-  "version": "69.3.0",
+  "version": "69.4.0",
   "description": "Noms JS SDK",
   "repository": "https://github.com/attic-labs/noms/tree/master/js/noms",
   "main": "dist/commonjs/noms.js",

js/noms/src/http-batch-store.js

@@ -121,8 +121,12 @@ export class Delegate {
     // Return success
     chunks.forEach(chunk => {
       const hashStr = chunk.hash.toString();
-      reqs[hashStr](chunk);
-      delete reqs[hashStr];
+      const req = reqs[hashStr];
+      // https://github.com/attic-labs/noms/issues/3133
+      if (req) {
+        req(chunk);
+        delete reqs[hashStr];
+      }
     });
 
     // Report failure