qapi-commands: Drop useless initialization

In generated command handlers, the assignment to retval dominates its
only use.  Therefore, its initialization is useless.  Drop it.
Suggested-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

docs/qapi-code-gen.txt

@@ -742,7 +742,7 @@ Example:
     static void qmp_marshal_input_my_command(QDict *args, QObject **ret, Error **errp)
     {
         Error *local_err = NULL;
-        UserDefOne *retval = NULL;
+        UserDefOne *retval;
         QmpInputVisitor *mi = qmp_input_visitor_new_strict(QOBJECT(args));
         QapiDeallocVisitor *md;
         Visitor *v;

scripts/qapi-commands.py

@@ -205,14 +205,10 @@ def gen_marshal_input(name, args, ret_type, middle_mode):
                 header=hdr)
 
     if ret_type:
-        if is_c_ptr(ret_type):
-            retval = "    %s retval = NULL;" % c_type(ret_type)
-        else:
-            retval = "    %s retval;" % c_type(ret_type)
         ret += mcgen('''
-%(retval)s
+    %(c_type)s retval;
 ''',
-                     retval=retval)
+                     c_type=c_type(ret_type))
 
     if len(args) > 0:
         ret += gen_visitor_input_containers_decl(args)