dotgen: squash -Wsign-conversion warnings in 'medians'

dotgen: squash -Wsign-conversion warnings in 'medians'

The compiler says:

  mincross.c: In function 'medians':
  mincross.c:1803:25: warning: conversion to 'size_t' {aka 'long unsigned int'}
    from 'int' may change the sign of the result [-Wsign-conversion]
   1803 |             qsort(list, j, sizeof(int), (qsort_cmpf) ordercmpf);
        |                         ^

By doing array indexing in this function universally as `size_t` we can avoid
this.