psbt: reject trailing final witness data

diff --git a/psbt/extractor.go b/psbt/extractor.go
index cf7e7b6..6d2d00f 100644
--- a/psbt/extractor.go
+++ b/psbt/extractor.go
@@ -46,24 +46,24 @@ func Extract(p *Packet) (*wire.MsgTx, error) {
 		// to extract that as well, parsing the lower-level transaction
 		// encoding.
 		if pInput.FinalScriptWitness != nil {
-			// In order to set the witness, need to re-deserialize
-			// the field as encoded within the PSBT packet.  For
-			// each input, the witness is encoded as a stack with
-			// one or more items.
+			// PSBT_IN_FINAL_SCRIPTWITNESS stores the complete
+			// scriptWitness for this transaction input. Decode that
+			// single serialized witness stack into TxWitness below.
 			witnessReader := bytes.NewReader(
 				pInput.FinalScriptWitness,
 			)
 
-			// First we extract the number of witness elements
-			// encoded in the above witnessReader.
+			// First extract the number of witness items encoded in
+			// the serialized scriptWitness.
 			witCount, err := wire.ReadVarInt(witnessReader, 0)
 			if err != nil {
 				return nil, err
 			}
 
-			// Now that we know how many inputs we'll need, we'll
-			// construct a packing slice, then read out each input
-			// (with a varint prefix) from the witnessReader.
+			// Allocate one slot per witness item, then read each
+			// varint-prefixed item from the witness value. The value
+			// must contain exactly this one stack, so the exhaustion
+			// check below rejects trailing bytes.
 			tin.Witness = make(wire.TxWitness, witCount)
 			for j := uint64(0); j < witCount; j++ {
 				wit, err := wire.ReadVarBytes(
@@ -75,6 +75,10 @@ func Extract(p *Packet) (*wire.MsgTx, error) {
 				}
 				tin.Witness[j] = wit
 			}
+
+			if err := assertFullyConsumed(witnessReader); err != nil {
+				return nil, err
+			}
 		}
 	}