PR 33223 ppc64: segfault on unknown relocation

PR 33223 ppc64: segfault on unknown relocation

Bounds check accesses to ppc64_elf_howto_table and don't dereference a
NULL howto.  I think this catches all cases where that might happen.

	PR 33223
bfd/
	* elf64-ppc.c (ppc64_elf_info_to_howto): Consolidate error handling.
	(ppc64_elf_check_relocs): Tidy error messages.
	(ppc64_elf_relocate_section): Don't segfault when attempting to
	report an unsupported relocation.  Don't pass a NULL howto to
	_bfd_clear_contents.
ld/
	* testsuite/ld-powerpc/elfv2-2so.d: Adjust to suit error message
	change.