Fix crash in debugger

lib/_debugger.js

@@ -291,7 +291,10 @@ Client.prototype.reqEval = function(expression, cb) {
     var frame = bt.frames[self.currentFrame];
 
     var evalFrames = frame.scopes.map(function(s) {
-      return bt.frames[s.index].index;
+      if (!s) return;
+      var x = bt.frames[s.index];
+      if (!x) return;
+      return x.index;
     });
 
     self._reqFramesEval(expression, evalFrames, cb);