Plugin/ImageInfo.pm: avoid warnings: Use of uninitialized value in lc at /usr/local/lib/perl5/site_perl/5.18.0/Mail/SpamAssassin/Plugin/ImageInfo.pm line 231.

git-svn-id: https://svn.apache.org/repos/asf/spamassassin/trunk@1507538 13f79535-47bb-0310-9956-ffa450edef68