runtime: fix another stack split bug
runtime: fix another stack split bug

Makes godoc --http=:1234 not crash on linux/amd64.

R=r
CC=golang-dev
http://codereview.appspot.com/1975044

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