apps/maze: fix possible stack overflow

The original code this app is inspired from uses rand(), which returns
a number >= 0.
mulberry32() on the other hand can return a negative number, in which
case will overflow the accesses in the dirs array. Fix that by forcing
the result to be unsigned.

apps/maze/maze.c

@@ -42,7 +42,7 @@ void carve(int x, int y) {
     int dirs[] = {0, 1, 2, 3};
     // Fisher-Yates shuffle
     for (int i = 3; i > 0; i--) {
-        int j = mulberry32() % (i + 1);
+        int j = (unsigned)mulberry32() % (i + 1);
         int tmp = dirs[i];
         dirs[i] = dirs[j];
         dirs[j] = tmp;