[BUGFIX] Use last result line of identify command

In GraphicalFunctions::imageMagickIdentify() the last line returned by
the executed command will be used as result.

This prevents the system from using possible error messages printed
before the actual result for further processing.

Resolves: #56574
Releases: 6.2
Change-Id: Ic179e0b68c7fa9a4f0f4b2342cb381c54fb740b9
Reviewed-on: https://review.typo3.org/28076
Reviewed-by: Xavier Perseguers
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind