internal/third_party/yaml: fix faulty decoding of hidden fields
Change-Id: Ib311a7c03caf08824bf16d16083940e80d5b6d18
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9841
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>