Use levelflat->name instead of flatname

Doesn't actually change anything (R_GetFlatForNum/R_CheckTextureNumForName handle this anyway). I just thought it's better to use the NUL terminated version since it's already there.

Might also be worth noting that levelflat->name is converted to all uppercase immediately after flatname is copied into it. However, this may not matter, since in practice we always use uppercase for flats and textures anyway.

merged

mentioned in commit 17616967