Don't crash if initializeVideo fails

Summary:
If avcodec_find_decoder returns NULL, a warning is printed and then NULL is
dereferenced later...

Test Plan:
A user crashed thumbnail.so reproducably with a specific file.
Doesn't anymore with this patch applied.

Reviewers: broulik

Reviewed By: broulik

Differential Revision: https://phabricator.kde.org/D16631