test: cover adjusting of last_with_datap in evbuffer_prepend() Before the fix: $ regress evbuffer/empty_reference_prepend.. evbuffer/empty_reference_prepend: [forking] FAIL ../test/regress_buffer.c:104: assert(chain == buf->first) FAIL ../test/regress_buffer.c:2291: Buffer format invalid [empty_reference_prepend FAILED] evbuffer/empty_reference_prepend_buffer: [forking] OK 1/2 TESTS FAILED. (0 skipped)