Prevent calling doaltarobj twice

My change to make items thrown by monsters landing on altar
caused doaltarobj being called twice when hero dropped an item
on it. Call the newer instance only when monsters are moving.

src/do.c

@@ -299,7 +299,8 @@ flooreffects(struct obj *obj, coordxy x, coordxy y, const char *verb)
             (void) obj_meld(&obj, &otmp);
         }
         res = (boolean) !obj;
-    } else if (IS_ALTAR(levl[x][y].typ) && cansee(x,y)) {
+    } else if (gc.context.mon_moving && IS_ALTAR(levl[x][y].typ)
+               && cansee(x,y)) {
         doaltarobj(obj);
     }