Fix bus error on Debian bullseye

Fix bus error on Debian bullseye

Problem reported by Lars Ingebrigtsen, and problem diagnosis
and most of this patch by Pip Cet (Bug#42832).
* src/pdumper.c (dump_bitsets_init): Rename from dump_bitset_init.
All callers changed.  Initialize two bitsets with a single malloc
call.
(struct pdumper_loaded_dump_private): New member last_mark_bits.
(pdumper_find_object_type_impl): Return PDUMPER_NO_OBJECT if
the last_mark_bits’ bit is clear.
(pdumper_set_marked_impl): Assert that the last_mark_bits’
bit is set.
(pdumper_clear_marks_impl): Save mark_bits into
last_mark_bits before clearing mark_bits.
Co-authored-by: Pip Cet <pipcet@gmail.com>