HLS: Fix timeout in hls_attribute_list_fuzzer

This CL fixes a timeout that can occur with the
hls_attribute_list_fuzzer if a generated attribute list has multiple
occurrences of the same attribute name. Previously this was an error
that could be skipped by resuming iteration (via `AttributeMap::Fill`),
but that was later changed to be a non-skippable error after the fuzz
test had been written.

Bug: 1287729
Change-Id: If8d500fc4cdb3e127865911ebe5b5f33fd12edb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3399315
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Will Cassella <cassew@chromium.org>
Cr-Commit-Position: refs/heads/main@{#960537}