drm/vmwgfx: avoid null_ptr_deref in vmw_framebuffer_surface_create_handle

In the Linux kernel, the following vulnerability has been resolved: 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.