Fix segfault when carried box explodes

Opening a trapped box in inventory, if the box exploded and was
not destroyed, then temporary variable otmp was pointing to null.

src/trap.c

@@ -6400,7 +6400,7 @@ chest_trap(
         bot(); /* to get immediate botl re-display */
     }
 
-    otmp->tknown = 1; /* hero knows chest is no longer trapped */
+    obj->tknown = 1; /* hero knows chest is no longer trapped */
     return FALSE;
 }