In disposition.c, remove the "=" from the loop termination expression:
bsh> diff -ed  ~/xgammon-0.98a.orig/decision.c decision.c
601c
    for (i=0; i<18;i++) other_array[i] /= sum;
.
588c
    for (i=0; i<18;i++) t_array[i] /= sum;
.

other_array[18] and t_array[18] shouldn't be referenced in the loops
abstracted above. The arrays are initialized from the endgame table in
an earlier loop from 0 to 17; which means the value at index 18 is
undefined. The program sets each array at index 18 to 0.0 before
proceeding to the winning chance calculation.