Fix XINFO help for unexpected options. (#9075)

Small cleanup and consistency.

src/t_stream.c

@@ -3459,6 +3459,11 @@ void xinfoCommand(client *c) {
 
     /* HELP is special. Handle it ASAP. */
     if (!strcasecmp(c->argv[1]->ptr,"HELP")) {
+        if (c->argc != 2) {
+            addReplySubcommandSyntaxError(c);
+            return;
+        }
+
         const char *help[] = {
 "CONSUMERS <key> <groupname>",
 "    Show consumers of <groupname>.",

tests/unit/type/stream.tcl

@@ -714,4 +714,9 @@ start_server {tags {"stream"}} {
         catch {r XGROUP help xxx} e
         assert_match "*Unknown subcommand or wrong number of arguments*" $e
     }
+
+    test {XINFO HELP should not have unexpected options} {
+        catch {r XINFO help xxx} e
+        assert_match "*Unknown subcommand or wrong number of arguments*" $e
+    }
 }