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.