Fix: problem when using custom processor with indexAnnotations()

Processor was throwing exception when indexAnnotations method was used
in custom processor.
This was hard to spot because of another problem with Maven which reports
success even though javac returned with 'Error' throwable.

Change-Id: I1db6196f444c61cef21ab320b718d39654be941f
Reviewed-on: https://pirx.atteo.com/gerrit/343
Tested-by: Mr. Jenkins
Reviewed-by: Tomek Piotrowski <tomek.piotrowski@atteo.com>