Fix infinite loop bug in previous commit

lib/sandbox.js

@@ -53,11 +53,12 @@ function runExtension(sessionContext, key, extCall, callback) {
   else {
     re[key] = [callback];
     extensions.run(extCall, function(err, fault, res) {
+      const callbacks = re[key];
+      delete re[key];
       if (!err && !fault)
         sessionContext.extensionsCache[key] = res;
-      for (let c of re[key])
+      for (let c of callbacks)
         c(err, fault);
-      delete re[key];
     });
   }
 }