partially revert afc2e8100a

Turns out that there are some signatures in the bitcoin blockchain that have
trailing 0s, for example
12a1b29fd6c295075b6a66f5fd90f0126ceb1fda4f15e4b44d92518bd52a5cdf has a signature
length of 0x45 where there are 0x47 bytes following that length check (one is
hashtype and is supposed to be trimmed out prior to calling the function). We
relax the paranoid length check to permit traling data, but not to permit
buffers that are too short. Change the test to passing with a big comment
stating why this is now considered a valid case.

signature.go

@@ -43,7 +43,7 @@ func ParseSignature(sigStr []byte, curve elliptic.Curve) (*Signature, error) {
 	// length of remaining message
 	siglen := sigStr[index]
 	index++
-	if int(siglen+2) != len(sigStr) {
+	if int(siglen+2) > len(sigStr) {
 		return nil, errors.New("malformed signature: bad length")
 	}
 	// trim the slice we're working on so we only look at what matters.

signature_test.go

@@ -151,7 +151,13 @@ var signatureTests = []signatureTest{
 			0x9d, 0x83, 0x1c, 0xc5, 0x6c, 0xbb, 0xac, 0x46, 0x22,
 			0x08, 0x22, 0x21, 0xa8, 0x76, 0x8d, 0x1d, 0x09, 0x01,
 		},
-		isValid: false,
+
+		// This test is now passing (used to be failing) because there
+		// are signatures in the blockchain that have trailing zero
+		// bytes before the hashtype. So ParseSignature was fixed to 
+		// permit buffers with trailing nonsense after the actual
+		// signature.
+		isValid: true,
 	},
 	signatureTest{
 		name: "X == N ",