Exiting a level with G_SetCustomExitVars plus skipstats keeps starpost values

What this means is that if this method is used to exit a level and the player has touched a starpost prior, they're gonna spawn in the wrong place in the next one.

This bug is already known and seems to hail from 2.1 but since it surfaced again in the OLDC it's now a Git issue too just to keep the memory fresh.