Merge branch 'savegame-hotfix' into 'next'

Savegame hotfix

This fixes the first mobj thinker added/read from $$$.sav not being relinked to other objects as their target/tracer/etc properly. In particular, this fixes Brak's electric shield disappearing for anyone joining a netgame playing ERCZ (as Brak happens to be the first object added/read in this case, and Brak is the shield's target).

See merge request !146