sidebar-thumbnails: connect to job finished signal before scheduling the job

taken from https://git.gnome.org/browse/evince/commit/?id=6b0630f8c2abd992d65075152dce5fefd9b51f45
ev-jobs: don't forget to unlock the trylock'ed mutex
ev-jobs: fix indentation a bit
epub backend: more anti-NULL protection for epub_document_set_index_pages
zero-init all signals arrays
va_start should be paired with va_end
removed some code that had no effect
don't leak memory