image/png: minor cleanup of a few tests

Removes a redundant err check and replaces some returns in a testing
loop with continue to prevent skipping unrelated test cases when
a failure is encountered.

Change-Id: Ic1a560751b95bb0ef8dfa957e057e0fa0c2b281d
Reviewed-on: https://go-review.googlesource.com/134236
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

src/image/png/reader_test.go

@@ -364,10 +364,6 @@ func TestReader(t *testing.T) {
 		}
 		defer sf.Close()
 		sb := bufio.NewScanner(sf)
-		if err != nil {
-			t.Error(fn, err)
-			continue
-		}
 
 		// Compare the two, in SNG format, line by line.
 		for {

src/image/png/writer_test.go

@@ -61,12 +61,12 @@ func TestWriter(t *testing.T) {
 		m1, err := readPNG(qfn)
 		if err != nil {
 			t.Error(fn, err)
-			return
+			continue
 		}
 		m2, err := encodeDecode(m1)
 		if err != nil {
 			t.Error(fn, err)
-			return
+			continue
 		}
 		// Compare the two.
 		err = diff(m0, m2)