evbuffer_add: Use last_with_datap if set, not last.

evbuffer_add: Use last_with_datap if set, not last.

evbuffer_add() would always put data in the last chain, even if there
was available space in a previous chain, and in doing so it also
failed to update last_with_datap, causing subsequent calls to other
functions that do look at last_with_datap to add data in the middle
of the evbuffer instead of at the end.

Fixes the evbuffer_add() part of issue #335, and the evbuffer/add2 and
evbuffer/add3 tests, and also prevents wasting space available in the
chain pointed to by last_with_datap.