spice-app: avoid crash when core spice module doesn't loaded

When qemu is built with modules, but a given module doesn't load
qemu should handle that gracefully. When ui-spice-core.so isn't
able to be loaded and qemu is invoked with -display spice-app or
-spice, qemu will dereference a null pointer. With this change we
check the pointer before dereferencing and error out in a normal
way.

Signed-off-by: Bruce Rogers <brogers@suse.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210213032318.346093-1-brogers@suse.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

ui/spice-app.c

@@ -129,6 +129,7 @@ static void spice_app_atexit(void)
 static void spice_app_display_early_init(DisplayOptions *opts)
 {
     QemuOpts *qopts;
+    QemuOptsList *list;
     GError *err = NULL;
 
     if (opts->has_full_screen) {
@@ -159,11 +160,16 @@ static void spice_app_display_early_init(DisplayOptions *opts)
             exit(1);
         }
     }
+    list = qemu_find_opts("spice");
+    if (list == NULL) {
+        error_report("spice-app missing spice support");
+        exit(1);
+    }
 
     type_register(&char_vc_type_info);
 
     sock_path = g_strjoin("", app_dir, "/", "spice.sock", NULL);
-    qopts = qemu_opts_create(qemu_find_opts("spice"), NULL, 0, &error_abort);
+    qopts = qemu_opts_create(list, NULL, 0, &error_abort);
     qemu_opt_set(qopts, "disable-ticketing", "on", &error_abort);
     qemu_opt_set(qopts, "unix", "on", &error_abort);
     qemu_opt_set(qopts, "addr", sock_path, &error_abort);