go.crypto/ssh: sanity check incoming packet length

The check for a sensible packet length was removed a while ago
when the window size and channel packet size checks were moved
into channel.go. While the RFC suggests that any packet of size
less than uint32 -1 is valid, most implmentations limit the size
to a smaller value. OpenSSH chose 256kb, so that sounds like a
sensible default.

R=agl, huin, kardianos
CC=golang-dev
https://golang.org/cl/6490098