libctf: Fix recursive descent into anonymous SOU fields

Approved by: re (gjb)
PR: 262412
Tested by: dhw, gallatin
Fixes: a6fb86917362 ("libctf: Handle CTFv3 containers")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 6e563a1b608438504d963c2d7c70e50d2e75af46)
(cherry picked from commit 8b01c3026b78152d9e0ca6f3758ea226f32e48b9)