Added null check for Elmah http handler.

When any registered instance of Elmah's IRequestAuthorizationHandler interface fails authorization or Elmah is not configured to allow remote access, it will set the status code to 403 and return null.

Calling ProcessRequest on a null handler causes a NullReferenceException which is logged and emailed.
Merge pull request #38 from bennor/patch-1

Added null check for Elmah http handler.