Don't segfault or assert on NULL tls_sec

Don't segfault or assert on NULL tls_sec

Real code won't hit these, but it's possible to contrive a testcase..

	* elf32-ppc.c (ppc_elf_relocate_section): Don't segfault on NULL
	tls_sec.
	* elf64-ppc.c (ppc64_elf_relocate_section): Likewise.
	* elflink.c (elf_link_output_extsym): Don't assert on NULL tls_sec.