test: regression for evbuffer_expand_fast_() with invalid last_with_datap Before the fix: $ regress --no-fork evbuffer/reserve_invalid_last_with_datap evbuffer/empty_chain_expand: [err] ../buffer.c:2138: Assertion chain == buf->first failed in evbuffer_expand_fast_ Aborted (core dumped) This is the a shorter version of test from the #806 (with some comments).