fix #2382 - temp Dex loss becomes permanent when dismounting

fix #2382 - temp Dex loss becomes permanent when dismounting

     From a bug report, if you dismount
from a steed whose legs are wounded, you won't recover the point of
dexterity which was removed when they becme injured.  He noticed the
case where the steed had just died, but it happens for voluntary
dismount too (actually any dismount except one which wounds the hero's
legs in the process:  DISMOUNT_THROWN or DISMOUNT_FELL).

     It seems rather odd that the hero temporarily loses Dex when the
steed becomes injured, but I haven't changed that.