Fix address violation problems when parsing corrupt ELF binaries.

Fix address violation problems when parsing corrupt ELF binaries.

	PR 21916
	* elf-attrs.c (_bfd_elf_parse_attributes): Complain about very
	small section lengths.
	* elf.c (_bfd_elf_setup_sections): Skip empty entries in the group
	table.
	(elfcore_grok_freebsd_prstatus): Add checks to make sure that
	there is enough data present in the note.