vrend: Add an assert for allocating the intermediate texture

With the format checks introduced earlier the double free bug reported
in #139 should no longer be possible. Nevertheless, add an assert to
ensure that this is checked.

Closes #139

v2: Update NDEBUG_UNUSED to MAYBE_UNUSED

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>