PR24236, Heap buffer overflow in _bfd_archive_64_bit_slurp_armap

PR24236, Heap buffer overflow in _bfd_archive_64_bit_slurp_armap

	PR 24236
	* archive64.c (_bfd_archive_64_bit_slurp_armap): Move code adding
	sentinel NUL to string buffer nearer to loop where it is used.
	Don't go past sentinel when scanning strings, and don't write
	NUL again.
	* archive.c (do_slurp_coff_armap): Simplify string handling to
	archive64.c style.