Add a missing break

Stepping on a magic trap while riding could delete the trap and
make the deleted trap hit the steed.

doc/fixes35.0

@@ -856,6 +856,7 @@ fix "object_is_local" panic when saving bones after hero is killed by explosion
 	produced by dropped or thrown lit potion of oil
 if lava burns up the player's water walking boots, the player falls in
 the messages for lava burning items up are always printed
+fix used-up magic trap trying to hit steed.
 
 
 Platform- and/or Interface-Specific Fixes