ld: Check archive only for archive member

ld: Check archive only for archive member

Since plugin_maybe_claim calls bfd_close on the original input BFD if it
isn't an archive member, pass NULL to bfd_plugin_close_file_descriptor
to indicate that the BFD isn't an archive member.

bfd/

	PR ld/18028
	* plugin.c (bfd_plugin_close_file_descriptor): Check archive
	only of abfd != NULL.
	(try_claim): Pass NULL to bfd_plugin_close_file_descriptor if
	it isn't an archive member.

ld/

	PR ld/18028
	* plugin.c (plugin_input_file): Add comments for abfd and ibfd.
	(plugin_object_p): Set input->ibfd to NULL if it isn't an
	archive member.