descriptor: InferScript, do not return top-level only func as sub descriptor

e.g. sh(addr(ADDR)) or sh(raw(HEX)) are invalid descriptors.

Making sh and wsh top level functions to return addr/raw descriptors when
the subscript inference fails.

src/script/descriptor.cpp

@@ -1717,6 +1717,10 @@ std::unique_ptr<DescriptorImpl> InferScript(const CScript& script, ParseScriptCo
         }
     }
 
+    // The following descriptors are all top-level only descriptors.
+    // So if we are not at the top level, return early.
+    if (ctx != ParseScriptContext::TOP) return nullptr;
+
     CTxDestination dest;
     if (ExtractDestination(script, dest)) {
         if (GetScriptForDestination(dest) == script) {