vtest: Allways check return value of vtest_block_write

Fixes Coverity ID: 1527228, 1527203 "Unchecked return value"

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/virgl/virglrenderer/-/merge_requests/975>
1 file changed