asan: NULL dereference in bfd_elf_set_group_contents

	* elf-bfd.h (struct output_elf_obj_tdata): Make num_section_syms
	unsigned.
	* elf.c (bfd_elf_set_group_contents): Bounds check sec->index
	and check that entry in elf_section_syms for sec is non-NULL.
	(_bfd_elf_symbol_from_bfd_symbol): Adjust.