[BUGFIX] Fatal Error in Page Tree filter

When entering short numeric values in the Page Tree filter of a large
tree, a PHP Fatal Error can occur:
Call to a member function offsetExists() on a non-object.

Change-Id: I3734769a068e442359dc69d4262fd237f9384860
Resolves: #28297
Releases: 4.6, 4.5
Reviewed-on: http://review.typo3.org/5729
Reviewed-by: Benny Schimmer
Tested-by: Benny Schimmer
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers