Fix segfault when first mobj in overlaycap is removed

When overlays are removed from the game through P_RemoveMobj, there's a small chance that the mobj is not removed properly from overlaycap, notably if it's in the first spot in the linked list. This causes the game to crash when P_RunOverlays is called afterwards, since the first value in overlaycap would not be valid anymore. The cause of this bug is due to a missing special check that checks the first entry in the list, as the iterator only checks the following value through hnext and thus misses to check the first value.