v3d: assert if v3d_compile returns NULL

v3d: assert if v3d_compile returns NULL

Ideally we would like to trigger a compilation error like we do on
v3dv (VK_ERROR_UNKNOWN). But with v3d we can't really do that, as this
could happen on a draw call. Let's at least assert so debug builds
stops at this point.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23203>