Fixes the bug that ward number is wrong on randart staves

There is a bug that when of warding and of greater warding both exit on a staff, the ward number is lower than predicted before changing element of the staff. The actual number is actually the higher of the sum of wards granted by of warding ego (2 * ego amount) and that granted by of greater warding ego (3 * ego amount). For example, a staff with 2 of warding egos and 1 of greater warding only has 4 wards at start, not 7. This commit fixes this bug.