advance_last_with_data() adjusts evbuffer.last_with_datap, and if we
will have empty chain in the middle advance_last_with_data() will stop,
while it should not, since while empty chains is not regular thing they
can pops up in various places like, and while I did not look through all
of them the most tricky I would say is:
evbuffer_reverse_space()/evbuffer_commit_space()
evbuffer_add_reference()
Test case from:
https://github.com/envoyproxy/envoy/pull/6062Fixes: #778
v2: keep last_with_datap really last with data, i.e. update only if
chain has data in it
5b19c9f62b