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.
Nick Mathewson
e0b7598833