tmfloorthing and tmhitthing are set to NULL at the start of P_MobjThinker,...

tmfloorthing and tmhitthing are set to NULL at the start of P_MobjThinker, preventing any weird cases of carrying over the previous mobj's floor object or such.

This fixes the issue with upside-down springs shooting downwards if you touch another of its kind. Also fixes one of the issues with monitors in Icicle Falls (after you phase inside the East-most float-bob FOF's monitor via the other bug and jump up to break it while there, the NEXT monitor moves upwards too)