encoding/pem: fix for TestFuzz, PEM type should not contain a colon

Fixes #22238

Change-Id: I8184f789bd4120f3e71c9374c7c2fcbfa95935bf
Reviewed-on: https://go-review.googlesource.com/132635Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/pem/pem_test.go

@@ -213,7 +213,9 @@ func TestFuzz(t *testing.T) {
 	}
 
 	testRoundtrip := func(block Block) bool {
-		if isBad(block.Type) {
+		// Reject bad Type
+		// Type with colons will proceed as key/val pair and cause an error.
+		if isBad(block.Type) || strings.Contains(block.Type, ":") {
 			return true
 		}
 		for key, val := range block.Headers {