[Bug] lib/arraystats: heap buffer overflow in disabled 'Discont' algorithm

[nilason]

Describe the bug

Shortly after introduction of GRASS arraystats library, a bug was discovered for the "Discont" algorithm, see ML, which was then disabled ac3098c.

Debugging this revealed a heap buffer overflow in line:

grass/lib/arraystats/class.c

Line 412 in bbb23c9

num[jj] = num[jj - 1];

The value of the variables at the AddressSanitizer breakpoint after running the equivalent to
v.class map=boundary_municp column=POP_APR_20 algo=dis nbclasses=5was:

tmp   (int)  0
j     (int)  1
i     (int)  5
jj    (int)  6
nff   (int)  7
num   (int*) 0x603000004a50
*num  (int)  -1094795586
nmax  (int)  3412

Note the size of num is 5 + 1:

grass/lib/arraystats/class.c

Line 299 in bbb23c9

num = G_malloc((nbclass + 1) * sizeof(int));

which obviously fails setting num[6] (num[5] is max).

I'm not familiar with the algorithm to present a fix for this.

@mlennert Perhaps you can revisit this now?