vl: fix "type is NULL" in -vga help

Don't pass NULL to module_object_class_by_name(), when the interface is
unavailable.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240715114420.2062870-1-marcandre.lureau@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

system/vl.c

@@ -1000,9 +1000,16 @@ static bool vga_interface_available(VGAInterfaceType t)
     const VGAInterfaceInfo *ti = &vga_interfaces[t];
 
     assert(t < VGA_TYPE_MAX);
-    return !ti->class_names[0] ||
-           module_object_class_by_name(ti->class_names[0]) ||
-           module_object_class_by_name(ti->class_names[1]);
+
+    if (!ti->class_names[0] || module_object_class_by_name(ti->class_names[0])) {
+        return true;
+    }
+
+    if (ti->class_names[1] && module_object_class_by_name(ti->class_names[1])) {
+        return true;
+    }
+
+    return false;
 }
 
 static const char *