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

wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old