elf.c and elflink.c fixes for commit 68bbe1183379

Plus some tidies to swap_out_syms.

	* elf.c (swap_out_syms): Handle NULL sym name.  Use correct type
	for return of _bfd_elf_strtab_add.  Simplify.
	* elflink.c (bfd_elf_match_symbols_in_sections): Handle NULL
	sym name.