net: never return -1 bytes read from netFD.Read
net: never return -1 bytes read from netFD.Read

If the a network read would block, and a packet arrived just before the timeout expired, then the number of bytes from the previous (blocking) read, -1, would be returned.

This change restores the previous logic, where n would be unconditionally set to 0 if err != nil, but was skipped due to a change in CL 6851096.

The test for this change is CL 6851061.

R=bradfitz, mikioh.mikioh, dvyukov, rsc
CC=golang-dev
http://codereview.appspot.com/6852085

rdp/rogerpack2005-golang,d0f/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,mhennings/marcohennings-go,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,d0f/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,webfd/go-zh,mhennings/marcohennings-go,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,bryanxu/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,rflanagan/reginaldflanagan-project1,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof