PR23199, Invalid SHT_GROUP entry leads to group confusion

PR23199, Invalid SHT_GROUP entry leads to group confusion

This patch prevents elf_next_in_group list pointer confusion when
SHT_GROUP sections specify other SHT_GROUP sections in their list of
group sections.

	PR 23199
	* elf.c (setup_group): Formatting.  Check that SHT_GROUP entries
	don't point at other SHT_GROUP sections.  Set shdr corresponding
	to invalid entry, to NULL rather than section 0.  Identify
	SHT_GROUP section index when reporting an error.  Cope with NULL
	shdr pointer.