ctf-archive sanity checks

Existing code checks that the first uint64_t ctfa_magic field is
available before reading but neglects to check that the last uint64_t
ctfa_ctfs is available before reading it in ctf_arc_bufpreamble.
ctf_arc_bufopen sets up a pointer to the struct ctf_archive in
ctf_new_archive_internal.  Extend the check to cover the entire struct.

	PR 33548
	PR 33549
	* ctf-archive.c (ctf_arc_bufpreamble, ctf_arc_bufopen): Check
	that buffer contains at least an entire struct ctf_archive
	when accessing such a struct.