image: add missing error check in test

Change-Id: Ia42a4a658e4207cc1f036f2faeac011e71edad77
GitHub-Last-Rev: b384f81799fcd404f53647d8a3015487ea240c6f
GitHub-Pull-Request: golang/go#30012
Reviewed-on: https://go-review.googlesource.com/c/go/+/160436
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/image/decode_test.go

@@ -123,6 +123,10 @@ loop:
 			continue
 		}
 		c, _, err := decodeConfig(it.filename)
+		if err != nil {
+			t.Errorf("%s: %v", it.filename, err)
+			continue loop
+		}
 		if m.ColorModel() != c.ColorModel {
 			t.Errorf("%s: color models differ", it.filename)
 			continue loop