[BUGFIX] Check if TER dump exists before modify time check

The modify time of the extensions.xml.gz file is used to display the last
update of the extension list. Checking if the file exists will prevent a
PHP warning.

Change-Id: Ic8cae6c591f0b6ff955fb01c192df9b17876fe68
Releases: 4.5
Resolves: #37946
Reviewed-on: http://review.typo3.org/21776
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>