transaction: range-check pegin data when parsing

It is possible to cause a panic by calling `TxIn::pegin_data` on a
malformed pegin, which has 6 entries and the last one (which should be a
Merkle proof of block inclusion) is too small to even have a blockhash
in it.

On the actual chain this should not be possible because invalid pegin
data would be rejected.

src/transaction.rs

@@ -436,6 +436,9 @@ impl<'tx> PeginData<'tx> {
         if pegin_witness.len() != 6 {
             return Err("size not 6");
         }
+        if pegin_witness[5].len() < 80 {
+            return Err("merkle proof too short");
+        }
 
         Ok(PeginData {
             outpoint: prevout,
@@ -2379,4 +2382,21 @@ mod tests {
             Err(encode::Error::ParseFailed("superfluous asset issuance")),
         ));
     }
+
+    #[test]
+    fn malformed_pegin() {
+        let mut input: TxIn = hex_deserialize!("\
+            0004000000000000ffffffff0000040000c0c0c0c0c0c0c0c0c0000000000000\
+            00805555555555555505c0c0c0c0c03fc0c0c0c0c0c0c0c0c0c0c0c00200ff01\
+            0000000000fd0000000000000000010000000000ffffffffffffffff00000000\
+            000000ff000000000000010000000000000000000001002d342d35313700\
+        ");
+        input.witness = hex_deserialize!("\
+            0000000608202020202020202020202020202020202020202020202020202020\
+            2020202020202020202020202020202020202020202020202020202020202020\
+            2020202020202020202020202020202020202020202020202020202020200000\
+            00000000000000000000000000000002000400000000\
+        ");
+        assert!(input.pegin_data().is_none());
+    }
 }