debugger: Fix bug in sb() with unnamed script

sb() cause error when unnamed script is loaded
<b>Reproduce:</b>
test.js
```javascript
//create some unnamed script
var f = Function("x", "y", "return x * y");
debugger;
```
node debug test
c
sb(2)
```
TypeError: Cannot call method 'indexOf' of undefined
    at _debugger.js:1377:45
    at Array.forEach (native)
    at Interface.setBreakpoint (_debugger.js:1376:28)
    at repl:1:2
    at Interface.controlEval (_debugger.js:969:21)
    at Interface.<anonymous> (repl.js:239:12)
    at Interface.EventEmitter.emit (events.js:95:17)
    at Interface._onLine (readline.js:202:10)
    at Interface._line (readline.js:531:8)
    at Interface._ttyWrite (readline.js:760:14)
```