Update core_scrapbook_display.php

Solve fatal error that *can* occur in automated job (Index Search Engine - All). Check if block (still) exists. 

Fatal error: Call to a member function getInstance() on a non-object in 8 core/controllers/blocks/core_scrapbook_display.php on line 32
Merge pull request #1363 from akodde/patch-2

Update core_scrapbook_display.php