image/png: fix compare-to-golden-file test.
image/png: fix compare-to-golden-file test.

bufio.Scanner.Scan returns whether the scan succeeded, not whether it
is done, so the test was mistakenly breaking early.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/93670045

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang