[BUGFIX] Check simplexml return type in ImageInfo

According to the php documentation[1] simplexml_load_file may return
false on failure. We need to check for this before we call the
attributes method on the expected SimpleXMLElement.

[1] http://php.net/manual/en/function.simplexml-load-file.php

Resolves: #79881
Releases: master, 8.7
Change-Id: Ic84710ab56796e9aafc5a85c8e41d7b08a676e0c
Reviewed-on: https://review.typo3.org/51735


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>