descriptor: don't assert success of extended key derivation

It might already fail, and we'll add another failure case.

Author: darosior

src/script/descriptor.cpp

@@ -388,14 +388,13 @@ class BIP32PubkeyProvider final : public PubkeyProvider
             }
         } else {
             for (auto entry : m_path) {
-                der = parent_extkey.Derive(parent_extkey, entry);
-                assert(der);
+                if (!parent_extkey.Derive(parent_extkey, entry)) return false;
             }
             final_extkey = parent_extkey;
             if (m_derive == DeriveType::UNHARDENED) der = parent_extkey.Derive(final_extkey, pos);
             assert(m_derive != DeriveType::HARDENED);
         }
-        assert(der);
+        if (!der) return false;
 
         final_info_out = final_info_out_tmp;
         key_out = final_extkey.pubkey;