[EPubGenerator] Reduce nesting level in loop

Summary:
The loop body is skipped when epub_it_get_curr(it) is invalid.
Make it more obvious there is no more code executed in this case.

Test Plan: no functional changes

Reviewers: Okular, ngraham

Reviewed By: ngraham

Subscribers: okular-devel

Tags: Okular

Differential Revision: https://phabricator.kde.org/D20946