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://golang.org/cl/93670045

src/pkg/image/png/reader_test.go

@@ -235,8 +235,8 @@ func TestReader(t *testing.T) {
 
 		// Compare the two, in SNG format, line by line.
 		for {
-			pdone := pb.Scan()
-			sdone := sb.Scan()
+			pdone := !pb.Scan()
+			sdone := !sb.Scan()
 			if pdone && sdone {
 				break
 			}