Repair breakage in early-error case of microdesc parsing

When I fixed #11243, I made it so we would take the digest of a
descriptor before tokenizing it, so we could desist from download
attempts if parsing failed.  But when I did that, I didn't remove an
assertion that the descriptor began with "onion-key".  Usually, this
was enforced by "find_start_of_next_microdescriptor", but when
find_start_of_next_microdescriptor returned NULL, the assertion was
triggered.

Fixes bug 16400.  Thanks to torkeln for reporting and
cypherpunks_backup for diagnosing and writing the first fix here.