virgl: don't send a shader create with no data. (v2)

This fixes the situation where we'd send a shader with just the
header and no data.

piglit/glsl-max-varyings test was causing this to happen, and
the renderer fix was breaking it.

v2: drop fprintf

Fixes: a8987b88 "virgl: add driver for virtio-gpu 3D (v2)"
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
(cherry picked from commit 240af614)