Tighten the check for pay-to-pubkey scripts.

This commit tightens the check for a pay-to-pubkey script by ensuring the
length of the pubkey is one of the two valid values of 33 or 65.  This
mirrors the checks in the multisig script type check as well.

ok @owainga

script.go

@@ -204,9 +204,9 @@ func isSmallInt(op *opcode) bool {
 // isPubkey returns true if the script passed is a pubkey transaction, false
 // otherwise.
 func isPubkey(pops []parsedOpcode) bool {
+	// valid pubkeys are either 33 or 65 bytes
 	return len(pops) == 2 &&
-		pops[0].opcode.value > OP_FALSE &&
-		pops[0].opcode.value <= OP_DATA_75 &&
+		(len(pops[0].data) == 33 || len(pops[0].data) == 65) &&
 		pops[1].opcode.value == OP_CHECKSIG
 }