Semi-hacky fix to R_AddSpriteDefs printing `Added -1 frames in 0 sprites`: if...

Semi-hacky fix to R_AddSpriteDefs printing `Added -1 frames in 0 sprites`: if the start marker is equal to or beyond the end marker, none of r_things' `for (l = startlump; l < endlump; l++)` loops will run, so just assume no sprites.

Simply put, `W_CheckNumForFolderStartPK3` and its `End` equivalent will never return INT16_MAX for nonexistence, and I don't want to break any assumptions elsewhere by modifying those functions directly, so this is the simplest possible fix.