[BUGFIX] Ensure variable imageSizes is an array before accessing its values

[BUGFIX] Ensure variable imageSizes is an array before accessing its values

It could be the PHP function "getimagesize" will fail to get the width and
the height of an image if the format is not supported by PHP. The patch
prevent accessing the value of an array which may be not an array.

Resolves: #64875
Releases: master, 6.2
Change-Id: Ibcbeb6edc7e23a2468cbcce40c2ecb0c2ba21142
Reviewed-on: http://review.typo3.org/36698
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>