PowerPC, sanity check r_offset in relocate_section

        * elf32-ppc.c (offset_in_range): New function.
	(ppc_elf_vle_split16): Sanity check r_offset before accessing
	section contents.  Return status.
        (ppc_elf_relocate_section): Sanity check r_offset before
        accessing section contents.  Don't segfault on NULL howto.