libctf: archive, open: when opening, always set errp to something

libctf: archive, open: when opening, always set errp to something

ctf_arc_import_parent, called by the cached-opening machinery used by
ctf_archive_next and archive-wide lookup functions like
ctf_arc_lookup_symbol, has an err-pointer parameter like all other opening
functions.  Unfortunately it unconditionally initializes it whenever
provided, even if there was no error, which can lead to its being
initialized to an uninitialized value.  This is not technically an
API-contract violation, since we don't define what happens to the error
value except when an error happens, but it is still unpleasant.

Initialize it only when there is an actual error, so we never initialize it
to an uninitialized value.

While we're at it, improve all the opening pathways: on success, set errp to
0, rather than leaving it what it was, reducing the likelihood of
uninitialized error param returns in callers too.  (This is inconsistent
with the treatment of ctf_errno(), but the err value being a parameter
passed in from outside makes the divergence acceptable: in open functions,
you're never going to be overwriting some old error value someone might want
to keep around across multiple calls, some of which are successful and some
of which are not.)

Soup up existing tests to verify all this.

Thanks to Bruce McCulloch for the original patch, and Stephen Brennan for
the report.

libctf/
	PR libctf/32903
	* ctf-archive.c (ctf_arc_open_internal): Zero errp on success.
	(ctf_dict_open_sections): Zero errp at the start.
	(ctf_arc_import_parent): Intialize err.
	* ctf-open.c (ctf_bufopen): Zero errp at the start.
	* testsuite/libctf-lookup/add-to-opened.c: Make sure one-element
	archive opens update errp.
	* testsuite/libctf-writable/ctf-compressed.c: Make sure real archive
	opens update errp.