Remove inocrrect panic from satisfy

It is true that all thresh fragments must be `d`, but in
certain edge cases it is possible that we don't have the
information to dissatify it. For example, the preimage to
the PkH fragment. Although users should always use descriptors
and DescriptorPublicKey type when dealing with this and this
would not be an issue in practise we should still remove the panic

Author: sanket1729

src/miniscript/satisfy.rs

@@ -627,9 +627,9 @@ impl Satisfaction {
         sat_indices.sort_by_key(|&i| {
             let stack_weight = match (&sats[i].stack, &ret_stack[i].stack) {
                 (&Witness::Unavailable, _) | (&Witness::Impossible, _) => i64::MAX,
-                (_, &Witness::Unavailable) | (_, &Witness::Impossible) => {
-                    unreachable!("Threshold fragments must be 'd'")
-                }
+                // This can only be the case when we have PkH without the corresponding
+                // Pubkey.
+                (_, &Witness::Unavailable) | (_, &Witness::Impossible) => i64::MIN,
                 (&Witness::Stack(ref s), &Witness::Stack(ref d)) => {
                     witness_size(s) as i64 - witness_size(d) as i64
                 }
@@ -669,7 +669,9 @@ impl Satisfaction {
         // For example, the fragment thresh(2, hash, hash, 0, 0)
         // is uniquely satisfyiable because there is no satisfaction
         // for the 0 fragment
-        else if !sats[sat_indices[k]].has_sig && sats[sat_indices[k]].stack != Witness::Impossible
+        else if k < sat_indices.len()
+            && !sats[sat_indices[k]].has_sig
+            && sats[sat_indices[k]].stack != Witness::Impossible
         {
             // All arguments should be `d`, so dissatisfactions have no
             // signatures; and in this branch we assume too many weak
@@ -744,9 +746,8 @@ impl Satisfaction {
         sat_indices.sort_by_key(|&i| {
             let stack_weight = match (&sats[i].stack, &ret_stack[i].stack) {
                 (&Witness::Unavailable, _) | (&Witness::Impossible, _) => i64::MAX,
-                (_, &Witness::Unavailable) | (_, &Witness::Impossible) => {
-                    unreachable!("Threshold fragments must be 'd'")
-                }
+                // This is only possible when one of the branches has PkH
+                (_, &Witness::Unavailable) | (_, &Witness::Impossible) => i64::MIN,
                 (&Witness::Stack(ref s), &Witness::Stack(ref d)) => {
                     witness_size(s) as i64 - witness_size(d) as i64
                 }