src: fix a couple of govet-discovered errors.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5021042

src/pkg/exp/regexp/exec_test.go

@@ -558,7 +558,7 @@ Reading:
 			}
 			have := re.FindStringSubmatchIndex(text)
 			if (len(have) > 0) != match {
-				t.Errorf("%s:%d: %#q.Match(%#q) = %v, but %#q.FindSubmatchIndex(%#q) = %v", file, lineno, pattern, text, match, text, have)
+				t.Errorf("%s:%d: %#q.Match(%#q) = %v, but %#q.FindSubmatchIndex(%#q) = %v", file, lineno, pattern, text, match, pattern, text, have)
 				continue Testing
 			}
 			if len(have) > len(pos) {

src/pkg/image/png/reader_test.go

@@ -262,7 +262,7 @@ func TestReaderError(t *testing.T) {
 			t.Errorf("decoding %s: %s, want %s", tt.file, err, tt.err)
 		}
 		if img != nil {
-			t.Errorf("decoding %s: have image + error")
+			t.Errorf("decoding %s: have image + error", tt.file)
 		}
 	}
 }