Missing check in isar_render_poses()

The function isar_render_poses() differs between BASOP and float in sanity check for memory allocation. The function was originally introduced under FIX_1119_SPLIT_RENDERING_VOIP.

Moreover, the the length of the flushbuffer is not correct in BASOP - it is twice as long wrt. the buffer in the float framework.

The functions should be aligned.