Merge pull request 'Check whether data is parsable by 'imagecreatefromstring'...

Merge pull request 'Check whether data is parsable by 'imagecreatefromstring' in jimIcon.' (#7) from wn/tt-rss:jimIcon-imagecreatefromstring into master

Reviewed-on: https://git.tt-rss.org/fox/tt-rss/pulls/7