contrib/vhost-user-gpu: avoid g_return_val_if() input validation

Do not validate input with g_return_val_if(). This API is intended for
checking programming errors and is compiled out with -DG_DISABLE_CHECKS.

Use an explicit if statement for input validation so it cannot
accidentally be compiled out.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20201118091644.199527-3-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

contrib/vhost-user-gpu/vhost-user-gpu.c

@@ -1044,7 +1044,9 @@ vg_get_config(VuDev *dev, uint8_t *config, uint32_t len)
 {
     VuGpu *g = container_of(dev, VuGpu, dev.parent);
 
-    g_return_val_if_fail(len <= sizeof(struct virtio_gpu_config), -1);
+    if (len > sizeof(struct virtio_gpu_config)) {
+        return -1;
+    }
 
     if (opt_virgl) {
         g->virtio_config.num_capsets = vg_virgl_get_num_capsets();