In case of failure in 'pdf_page_init', the mupdf_page is being cleared
and freed but the zathura_page still holds a pointer to it, resulting
in a double free when 'pdf_page_clear' is later called.

The execution flow resulting in a double free is the following:
  zathura_page_new()
    pdf_page_init()
      zathura_page_set_data()
      failure of fz_load_page(),fz_bound_page() or fz_new_stext_sheet()
      pdf_page_clear()
    zathura_page_free()
      pdf_page_clear() // double free

Make sure 'zathura_page_set_data' is called only if no errors occurred
so that page->data never points to freed memory.