added assert for usage of uninitialized stack memory in n64_rdp::SpanDraw1Cycle() (nw)

happens with e.g. n64 -cart vprowrs2 - it actually already goes out of
bounds in poly_manager::render_triangle_custom() but since that code is
generic it's hard to add an assert there / also added another FBSize
check
Merge https://github.com/mamedev/mame