net/rpc: improve response reading logic
net/rpc: improve response reading logic

CL 5956051 introduced too many call != nil checks, so
attempt to improve this by splitting logic into three
distinct parts.

R=r
CC=golang-dev
http://codereview.appspot.com/6248048

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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