gc: do not crash on bad [...]T
gc: do not crash on bad [...]T

Fixes issue 879.

R=ken2
CC=golang-dev
http://codereview.appspot.com/1678048

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