GBookmarkFile: nullptr access in current_element

==13==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000
==13==The signal is caused by a READ memory access.
==13==Hint: address points to the zero page.
SCARINESS: 10 (null-deref)
    #0 0x5efd91 in current_element glib/gmarkup.c:881:30
    #1 0x5f22de in g_markup_parse_context_end_parse glib/gmarkup.c
    #2 0x5bee8f in g_bookmark_file_parse glib/gbookmarkfile.c:1431:15
    #3 0x5bebca in g_bookmark_file_load_from_data glib/gbookmarkfile.c:1649:12

4-bytes test case.