gc: fix silent sign truncation in pgen.c
gc: fix silent sign truncation in pgen.c

Fixes issue 2076.

R=golang-dev, r, r, dsymonds, lucio.dere
CC=golang-dev
http://codereview.appspot.com/4744047

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

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