Merge #740: fix: zero sequence check on relative time

c128e79 fix: zero sequence check on relative time (Chris Hyunhum Cho)

Pull request description:

  I've just found out RelLockTime misses zero sequence check, which must be disallowed for miniscript.

ACKs for top commit:
  apoelstra:
    ACK c128e79 successfully ran local tests

Tree-SHA512: 9ccc96d67672c2077ec20efeda105e9941a1318030ac63b46e3260b886605d8a7231176413b4dccb6f1542008890f7df977bf4ed445f0fa9550cfa8f8dfaa0e7

src/primitives/relative_locktime.rs

@@ -65,7 +65,7 @@ impl RelLockTime {
 impl convert::TryFrom<Sequence> for RelLockTime {
     type Error = RelLockTimeError;
     fn try_from(seq: Sequence) -> Result<Self, RelLockTimeError> {
-        if seq.is_relative_lock_time() {
+        if seq.is_relative_lock_time() && seq != Sequence::ZERO {
             Ok(RelLockTime(seq))
         } else {
             Err(RelLockTimeError { value: seq.to_consensus_u32() })