Bug 1050272 - Fix 'function does not always return a value' warnings in ShumwayStreamConverter.jsm

Fixes several warnings such as:
```
System JS : WARNING resource://shumway/ShumwayStreamConverter.jsm:154 - anonymous function does not always return a value
System JS : WARNING resource://shumway/ShumwayStreamConverter.jsm:136 - function fetchPolicyFile does not always return a value
```
`return` shall be present after the callback call