bfd: Set error to bfd_error_malformed_archive only if unset

When reading an archive member, set error to bfd_error_malformed_archive
on open_nested_file failure only if the error is unset.

	PR ld/28138
	* archive.c (_bfd_get_elt_at_filepos): Don't set error to
	bfd_error_malformed_archive if it has been set.