miniscript: make GetStackSize independent of P2WSH context

It was taking into account the P2WSH script push in the number of stack
elements.

Author: darosior

src/script/miniscript.h

@@ -1148,16 +1148,16 @@ struct Node {
         return true;
     }
 
-    /** Return the maximum number of stack elements needed to satisfy this script non-malleably, including
-     * the script push. */
+    /** Return the maximum number of stack elements needed to satisfy this script non-malleably.
+     * This does not account for the P2WSH script push. */
     std::optional<uint32_t> GetStackSize() const {
         if (!ss.sat.valid) return {};
-        return ss.sat.value + 1;
+        return ss.sat.value;
     }
 
     //! Check the maximum stack size for this script against the policy limit.
     bool CheckStackSize() const {
-        if (const auto ss = GetStackSize()) return *ss - 1 <= MAX_STANDARD_P2WSH_STACK_ITEMS;
+        if (const auto ss = GetStackSize()) return *ss <= MAX_STANDARD_P2WSH_STACK_ITEMS;
         return true;
     }
 

src/test/fuzz/miniscript.cpp

@@ -973,7 +973,7 @@ void TestNode(const NodeRef& node, FuzzedDataProvider& provider)
 
     if (nonmal_success) {
         // Non-malleable satisfactions are bounded by GetStackSize().
-        assert(witness_nonmal.stack.size() <= *node->GetStackSize());
+        assert(witness_nonmal.stack.size() <= *node->GetStackSize() + 1);
         // If a non-malleable satisfaction exists, the malleable one must also exist, and be identical to it.
         assert(mal_success);
         assert(witness_nonmal.stack == witness_mal.stack);