Guard against error

Error, repro not yet clear

```
TypeError: Cannot read property '0' of undefined
    at /home/ubuntu/public-website/node_modules/clarify/clarify.js:10:35
    at Array.0 (/home/ubuntu/public-website/node_modules/clarify/clarify.js:9:17)
    at TraceModifier._modify (/home/ubuntu/public-website/node_modules/clarify/node_modules/stack-chain/stack-chain.js:41:32)
    at Function.Error.prepareStackTrace (/home/ubuntu/public-website/node_modules/clarify/node_modules/stack-chain/stack-chain.js:81:25)
```

clarify.js

@@ -7,6 +7,7 @@ var chain = require('stack-chain');
 
 chain.filter.attach(function (error, frames) {
   return frames.filter(function (callSite) {
-    return (callSite.getFileName()[0] === '/');
+    var name = callSite && callSite.getFileName();
+    return (name && name[0] === '/');
   });
 });