vms buffer overflows and large memory allocation

	* vms-lib.c (struct carsym_mem): Add limit.
	(vms_add_index): Heed limit.
	(vms_traverse_index): Catch buffer overflows.  Remove outdated fixme.
	(vms_lib_read_index): Set up limit.  Catch 32-bit overflow.
	Always return actual number read.
	(_bfd_vms_lib_archive_p): Catch buffer overflows.  Replace
	assertion with error exit.