Add clean up to K_PuntMine

This cleans up hnext when a mine shield is P_RemoveMobj'ed while being punted
Otherwise, hnext could point to a removed mobj and cause undefined behavior
This also fixes the bug where if you have multiple mines in your slot, drag one
and have it punted, all your unused mines would disappear.
This should/may fix the crashes/desyncs I've observed in gameplay when a held mine is punted.