fix M159 - crash at end of prayer

fix M159 - crash at end of prayer

     Fix the crash caused by division by zero (attempt to compute rn2(0))
when deciding prayer boon for a character whose Luck went negative during
the course of the prayer.  <email deleted> triggered it
by killing a shopkeeper with the ongoing damage from a scroll of stinking
cloud; his non-chaotic character was branded a murderer and lost two points
of Luck after the prayer was already in progress.  (Prayers fail when Luck
is already negative, so the code to pick a boon expects non-negative values;
the fact that is always adds at least +2 leads to me to suspect that someone
already realized that luck timeout on Friday 13th could result in Luck of -1
at the end of a successful prayer--that value doesn't trigger this crash.)