Buffer overrun in cosmology.c

Running --cosmology with the GCC sanitizer enabled and it came across an overflow at line 781 of src/cosmology.c, i.e:

    if (i_a != cosmology_table_length) {
*      scale = r_interp - c->comoving_distance_interp_table[i_a - 1];
       scale /= c->comoving_distance_interp_table[i_a] -
                c->comoving_distance_interp_table[i_a - 1];

Turns out the issue is an underflow when i_a == 0, since we index by i_a - 1.

Applied a simple fix of:

    if (i_a < cosmology_table_length && i_a > 0) {

which should protect against over and underruns, but since I've no idea if this is a critical change over to you.

(btw, sanitizer calls this an overflow to the left, hence the confusing description of underflow).