Reset the available read buffers if rbytes==0

If rbytes is zero then we don't need any of the previous data between
rbuf (start of read buffer) and rcurr (current position), so can reset
rcurr back to rbuf, making more read buffer available without having
to allocate more memory.

Change-Id: I5850a4de853687db84d89123335cc56141329b76
Reviewed-on: http://review.couchbase.org/38553
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>