libctf: allow ctf_arc_bufpreamble to fail

libctf: allow ctf_arc_bufpreamble to fail

The recent libctf fix for ctf_arc_bufpreamble missed a case:
what if the input is exactly sizeof (ctf_archive_t) in size (which can
happen if the archive has no members at all, so returning the preamble
from one of the members is in any case impossible?).  In this case
it'll return an off-the-end pointer, and its caller will overrun.

Allow it to fail in this case, returning NULL, and adjust its sole
caller.  The caller's conclusions in this case will be wrong (it will
conclude that the archive is connected to .symtab), but the incorrect
conclusions are harmless because the lack of archive members will
immediately cause a failure in ctf_arc_bufopen(), and an error return.

Thanks to Alan Modra for the original fix this soups up.

libctf/
	* ctf-archive.c (ctf_arc_bufpreamble): Fail if the archive is
	too short (or empty, with no dicts to contain preambles),
	returning NULL.
	* ctf-open-bfd.c (ctf_bfdopen_ctfsect): Handle a NULL return.