image/gif: simplify blockReader.Read. Inverting the tests avoids recursion and simplifies the flow.
image/gif: simplify blockReader.Read.
Inverting the tests avoids recursion and simplifies the flow.

R=nigeltao
CC=golang-dev
http://codereview.appspot.com/4551057

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