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.
(cherry picked from commit
4bba0fb1c6d391a217c25e44398a7e1c7090155f)