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

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,ssrl/go,dsriram/golang,apparentlymart/golang,dsriram/golang,gnanderson/go,gnanderson/go,apparentlymart/golang,ssrl/go,pombredanne/go-deleteme,ssrl/go,apparentlymart/golang,dsriram/golang,dsriram/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,tv42/old-go,ssrl/go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,ssrl/go,gnanderson/go,dsriram/golang,apparentlymart/golang,tv42/old-go,ssrl/go,apparentlymart/golang,apparentlymart/golang,ssrl/go,pombredanne/go-deleteme