descriptor: reject strings of the form "tr(<key>,)"

Fixes #736

Author: apoelstra

src/descriptor/mod.rs

@@ -2011,6 +2011,14 @@ pk(03f28773c2d975288bc7d1d205c3748651b075fbc6610e58cddeeddf8f19405aa8))";
         Descriptor::<DescriptorPublicKey>::from_str("wsh(andor(pk(tpubDEN9WSToTyy9ZQfaYqSKfmVqmq1VVLNtYfj3Vkqh67et57eJ5sTKZQBkHqSwPUsoSskJeaYnPttHe2VrkCsKA27kUaN9SDc5zhqeLzKa1rr/0'/<0;1;2;3>/*),older(10000),pk(tpubD8LYfn6njiA2inCoxwM7EuN3cuLVcaHAwLYeups13dpevd3nHLRdK9NdQksWXrhLQVxcUZRpnp5CkJ1FhE61WRAsHxDNAkvGkoQkAeWDYjV/8/<0;1;2>/*)))").unwrap_err();
     }
 
+    #[test]
+    fn regression_736() {
+        Descriptor::<DescriptorPublicKey>::from_str(
+            "tr(0000000000000000000000000000000000000000000000000000000000000002,)",
+        )
+        .unwrap_err();
+    }
+
     #[test]
     fn regression_734() {
         Descriptor::<DescriptorPublicKey>::from_str(

src/descriptor/tr.rs

@@ -603,9 +603,6 @@ fn parse_tr_tree(s: &str) -> Result<expression::Tree, Error> {
             return Err(Error::Unexpected("invalid taproot internal key".to_string()));
         }
         let internal_key = expression::Tree { name: key.name, args: vec![] };
-        if script.is_empty() {
-            return Ok(expression::Tree { name: "tr", args: vec![internal_key] });
-        }
         let (tree, rest) = expression::Tree::from_slice_delim(script, 1, '{')?;
         if rest.is_empty() {
             Ok(expression::Tree { name: "tr", args: vec![internal_key, tree] })