openpgp: Fix panic on v3 public keys with too-short modulus.

Found using gofuzz.

Fixes golang/go#11504

Change-Id: I49cf01e75e37c5d87dad58c5349161d79d0b72f5
Reviewed-on: https://go-review.googlesource.com/12635
Reviewed-by: Adam Langley <agl@golang.org>

openpgp/packet/public_key_v3.go

@@ -95,6 +95,11 @@ func (pk *PublicKeyV3) parseRSA(r io.Reader) (err error) {
 		return
 	}
 
+	// RFC 4880 Section 12.2 requires the low 8 bytes of the
+	// modulus to form the key id.
+	if len(pk.n.bytes) < 8 {
+		return errors.StructuralError("v3 public key modulus is too short")
+	}
 	if len(pk.e.bytes) > 3 {
 		err = errors.UnsupportedError("large public exponent")
 		return