PR 33473 SEGV in _bfd_elf_gc_mark_debug_special_section_group

The code that faulted made the assumption that a group section always
had at least one valid member.  Fix that assumption.  Also fail if all
entries in a SHT_GROUP section are invalid.  (An empty group will not
result in a call to process_sht_group_entries.)

	PR 33473
	* elflink.x (_bfd_elf_gc_mark_debug_special_section_group): Don't
	segfault on empty group.
	* elf.c (process_sht_group_entries): Return false if all
	entries are invalid.