net/rpc: shut down connection if gob has error

net/rpc: shut down connection if gob has error
The nicest solution would be to buffer the message and only write
it if it encodes correctly, but that adds considerable memory and
CPU overhead for a very rare condition. Instead, we just shut
down the connection if this happens.
Fixes #7689.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/146670043