encoding/pem: properly calculate end indexes

When a block is missing the END line trailer, calculate the indexes of
the end and end trailer _before_ continuing the loop, making the
reslicing at the start of the loop work as expected.

Change-Id: If45c8cb473315623618f02cc7609f517a72d232d
Reviewed-on: https://go-review.googlesource.com/c/go/+/714200
Auto-Submit: Roland Shoemaker <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: rolandshoemaker

src/encoding/pem/pem.go

@@ -116,11 +116,11 @@ func Decode(data []byte) (p *Block, rest []byte) {
 		var typeLine []byte
 		var consumed int
 		typeLine, rest, consumed = getLine(rest)
+		endIndex -= consumed
+		endTrailerIndex -= consumed
 		if !bytes.HasSuffix(typeLine, pemEndOfLine) {
 			continue
 		}
-		endIndex -= consumed
-		endTrailerIndex -= consumed
 		typeLine = typeLine[0 : len(typeLine)-len(pemEndOfLine)]
 
 		p = &Block{

src/encoding/pem/pem_test.go

@@ -736,3 +736,7 @@ func FuzzDecode(f *testing.F) {
 		Decode(data)
 	})
 }
+
+func TestMissingEndTrailer(t *testing.T) {
+	Decode([]byte{0x2d, 0x2d, 0x2d, 0x2d, 0x2d, 0x42, 0x45, 0x47, 0x49, 0x4e, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0x20, 0xa, 0x2d, 0x2d, 0x2d, 0x2d, 0x2d, 0x45, 0x4e, 0x44, 0x20})
+}