Merge bitcoin#20761: fuzz: Check that NULL_DATA is unspendable

fa26303 fuzz: Check that NULL_DATA is unspendable (MarcoFalke)

Pull request description:

  * Every script of type NULL_DATA must be unspendable
  * The only know types of unspendable scripts are NULL_DATA and certain NONSTANDARD scripts

ACKs for top commit:
  sipa:
    utACK fa26303

Tree-SHA512: 8297fbacf32b4868b12accc1c052d352d02d96540a1fc883de9d04a3df8734116deecc33046495c9a3af6d79fec7f8d63afbfa5e401a2ca8d7c70f0f13735c0d

src/test/fuzz/script.cpp

@@ -69,6 +69,13 @@ FUZZ_TARGET_INIT(script, initialize_script)
 
     TxoutType which_type;
     (void)IsStandard(script, which_type);
+    if (which_type == TxoutType::NULL_DATA) {
+        assert(script.IsUnspendable());
+    }
+    if (script.IsUnspendable()) {
+        assert(which_type == TxoutType::NULL_DATA ||
+               which_type == TxoutType::NONSTANDARD);
+    }
 
     (void)RecursiveDynamicUsage(script);
 
@@ -77,7 +84,6 @@ FUZZ_TARGET_INIT(script, initialize_script)
 
     (void)script.IsPayToScriptHash();
     (void)script.IsPushOnly();
-    (void)script.IsUnspendable();
     (void)script.GetSigOpCount(/* fAccurate= */ false);
 
     (void)FormatScript(script);