fix getStrongholdPieces going out of bounds

[lithodoradiffusa]

I found that getStrongholdPieces can return more pieces than the buffer size n, causing memory issues

env.nmax is set but never used.
this pr adds a check for *env->n >= env->nmax at the end of addStrongholdPiece.

Here is example code to recreate this:
It prints:
"remaining sh: 128
Generated 109 Pieces. max pieces = 100"
then crashes as memory for Generator g is corrupted.

#include "./cubiomes/features/stronghold.h"
#include "./cubiomes/finders.h"
#include "./cubiomes/biomes.h"
#include "./cubiomes/generator.h"

#include <stdint.h>
#include <stdio.h>

int main() {

  int nmax = 100;
  int mc = MC_NEWEST;
  Generator g;
  setupGenerator(&g, mc, 0);
  StructureSaltConfig ssc;
  StrongholdIter sh;
  uint64_t seed = 8833883388338848ull;
  Pos pos = initFirstStronghold(&sh, mc, seed);

  applySeed(&g, DIM_OVERWORLD, seed);
  int sh_remain = 128;
  while (sh_remain) {
    sh_remain = nextStronghold(&sh, &g);
    printf("remaining sh: %d\n",sh_remain);
    Piece list[nmax];
    int chunkX = sh.pos.x >> 4;
    int chunkZ = sh.pos.z >> 4;
    int biomeId = getBiomeAt(&g, 1, sh.pos.x, 256, sh.pos.z);
    int err_ssc = getStructureSaltConfig(Stronghold, mc, biomeId, &ssc);
    int num_pieces =
        getStrongholdLoot(list, nmax, ssc, mc, seed, chunkX, chunkZ);
    printf("Generated %d Pieces. max pieces = %d\n", num_pieces, nmax);
  }
  return 0;
}

[xpple]

Thanks!