descriptor: refuse to parse unspendable miniscript descriptors

It's possible for some unsatisfiable miniscripts to be considered sane.
Make sure we refuse to import those, as they would be unspendable.

src/script/descriptor.cpp

@@ -1541,14 +1541,14 @@ std::unique_ptr<DescriptorImpl> ParseScript(uint32_t& key_exp_index, Span<const
                 error = std::move(parser.m_key_parsing_error);
                 return nullptr;
             }
-            if (!node->IsSane()) {
+            if (!node->IsSane() || node->IsNotSatisfiable()) {
                 // Try to find the first insane sub for better error reporting.
                 auto insane_node = node.get();
                 if (const auto sub = node->FindInsaneSub()) insane_node = sub;
                 if (const auto str = insane_node->ToString(parser)) error = *str;
                 if (!insane_node->IsValid()) {
                     error += " is invalid";
-                } else {
+                } else if (!node->IsSane()) {
                     error += " is not sane";
                     if (!insane_node->IsNonMalleable()) {
                         error += ": malleable witnesses exist";
@@ -1561,6 +1561,8 @@ std::unique_ptr<DescriptorImpl> ParseScript(uint32_t& key_exp_index, Span<const
                     } else if (!insane_node->ValidSatisfactions()) {
                         error += ": needs witnesses that may exceed resource limits";
                     }
+                } else {
+                    error += " is not satisfiable";
                 }
                 return nullptr;
             }

src/script/miniscript.h

@@ -1161,6 +1161,9 @@ public:
         return true;
     }
 
+    //! Whether no satisfaction exists for this node.
+    bool IsNotSatisfiable() const { return !GetStackSize(); }
+
     //! Return the expression type.
     Type GetType() const { return typ; }