Fix use-after-frees around mobjs

Fixes a series of use-after-frees related to mobjs. Most notably, makes R_RemoveMobjInterpolator properly remove the last item of interpolated_mobjs, and fixes every place where target, tracer, hnext or hprev were modified without proper reference counting – including one in p_saveg.c.

A notable use-after-free that this fixes is one that happens when a player spectates. While I cannot test this myself, I have heard that spectating mid-race can cause desyncs – this has a good chance of fixing them.