This CL fixes two issues sending data to the remote peer.
The first bug occurs when the size of the buffer passed to
Write is larger than the current window, in this case, w.rwin
can become negative.

The second issue is more problematic than the first as the
amount of data passed to writePacket was not limited to w.rwin.
In this case the remote peer could silently drop the additional
data, or drop the connection.

Credit to Jacek Masiulaniec for the bug report.

R=agl, jacek.masiulaniec
CC=golang-dev
https://golang.org/cl/5511043