Fix HWR_ProjectSprite to check properly whether the displayed player's mobj or its subsector exists, to avoid a crash when checking for fake planes. (also use viewplayer since its available to use, silly hardware code)

Also tweaked a weird splitscreen check in HWR_DrawSpriteShadow; still investigating whether stplyr is ever not player 2 when it's player 2's view, but this looks better for now