fix #H6285 - flooreffects and deltrap panic

fix #H6285 - flooreffects and deltrap panic

Reported 14 months ago, a monster reading a scroll of earth which
dropped a boulder that killed another monster in an adjacent pit
was giving credit/blame to the hero and could also trigger a panic.
If the monster was killed, the pit would be filled and deleted via
m_detach and then when flooreffects tried to delete the same trap,
it accessed freed memory and deltrap could panic.