Bugfix: make ToPrivateString work with x-only keys

Author: sipa

src/script/descriptor.cpp

@@ -259,7 +259,15 @@ class ConstPubkeyProvider final : public PubkeyProvider
     bool ToPrivateString(const SigningProvider& arg, std::string& ret) const override
     {
         CKey key;
-        if (!arg.GetKey(m_pubkey.GetID(), key)) return false;
+        if (m_xonly) {
+            for (const auto& keyid : XOnlyPubKey(m_pubkey).GetKeyIDs()) {
+                arg.GetKey(keyid, key);
+                if (key.IsValid()) break;
+            }
+        } else {
+            arg.GetKey(m_pubkey.GetID(), key);
+        }
+        if (!key.IsValid()) return false;
         ret = EncodeSecret(key);
         return true;
     }