[BUGFIX] Remove PHP warnings when no icon is found in FormEngine

If running in development mode and an icon
in FormEngine is not found, TYPO3 shows
the PHP E_WARNING of getimagesize() in a flash message.

The check should therefore be done in a cleaner way,
without the @ symbol.

Releases: 6.3, 6.2
Resolves: #61394
Change-Id: I546faee95bf304e135709c3e1c95292f48daf3d7
Reviewed-on: http://review.typo3.org/32625
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>