Fix crash when clicking empty canvas while loading.

The PAGE_CACHE_LEN macro erroneously evaluates to 1 in the pixbuf_cache structure's freshly initialised state (start_page, end_page are -1). This causes a segfault (observed idly clicking the canvas while loading a big DJVU file) when `ev_pixbuf_cache_set_selection_list` calls `clear_job_selection(pixbuf_cache->job_list + 0)`.