drm/vmwgfx: avoid null_ptr_deref in vmw_framebuffer_surface_create_handle

The 'vmw_user_object_buffer' function may return NULL with incorrect
inputs. To avoid possible null pointer dereference, add a check whether
the 'bo' is NULL in the vmw_framebuffer_surface_create_handle.

Fixes: d6667f0d ("drm/vmwgfx: Fix handling of dumb buffers")
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241029083429.1185479-1-chenridong@huaweicloud.com