nouveau/video: wrap assertion within #ifndef NDEBUG

nouveau/video: wrap assertion within #ifndef NDEBUG

The change is necessary to avoid the following building error in android:

external/mesa/src/gallium/drivers/nouveau/nouveau_vp3_video_bsp.c: In function 'nouveau_vp3_bsp_next':
external/mesa/src/gallium/drivers/nouveau/nouveau_vp3_video_bsp.c:269:14: error: 'bsp_bo' undeclared (first use in this function)
       assert(bsp_bo->size >= str_bsp->w0[0] + num_bytes[i]);
              ^
This matches the declaration of the variables in question.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>