gofmt: don't substitute invalid positions with valid ones in rewrites
gofmt: don't substitute invalid positions with valid ones in rewrites

Fixes rewrite bug: 'f(x) -> f(0)' where functions "picked up" ... arguments.

R=rsc
CC=golang-dev
http://codereview.appspot.com/2279041

mhennings/marcohennings-go,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,scirelli/scirelli-go,mhennings/marcohennings-go,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,npe9/noahevans-go9,bryanxu/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,npe9/noahevans-go9,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,npe9/noahevans-go9,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,scirelli/scirelli-go,scirelli/scirelli-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,npe9/noahevans-go9,bryanxu/go-zh,npe9/noahevans-go9,d0f/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,npe9/noahevans-go9,webfd/go-zh,d0f/go-zh,d0f/go-zh,rflanagan/reginaldflanagan-project1,webfd/go-zh,scirelli/scirelli-go,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,mhennings/marcohennings-go,npe9/noahevans-go9,d0f/go-zh,webfd/go-zh,Triskite/willstone-goclone,Triskite/willstone-goclone,d0f/go-zh,scirelli/scirelli-go,bryanxu/go-zh,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,webfd/go-zh,scirelli/scirelli-go,webfd/go-zh,mhennings/marcohennings-go,npe9/noahevans-go9,d0f/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof