fix: save execution stack in query as string

Backport #15039 to 6.x

Author: vkarpov15

lib/helpers/query/wrapThunk.js

@@ -21,10 +21,10 @@ module.exports = function wrapThunk(fn) {
         str = str.slice(0, 60) + '...';
       }
       const err = new MongooseError('Query was already executed: ' + str);
-      err.originalStack = this._executionStack.stack;
+      err.originalStack = this._executionStack;
       return cb(err);
     }
-    this._executionStack = new Error();
+    this._executionStack = new Error().stack;
 
     fn.call(this, cb);
   };