debugger: prevent crash when backtrace doesn't come back

lib/_debugger.js

@@ -261,6 +261,12 @@ Client.prototype.reqEval = function(expression, cb) {
 
   // Otherwise we need to get the current frame to see which scopes it has.
   this.reqBacktrace(function(bt) {
+    if (!bt.frames) {
+      // ??
+      cb({});
+      return;
+    }
+
     var frame = bt.frames[self.currentFrame];
 
     var evalFrames = frame.scopes.map(function(s) {