Try to fix memory leaks

[Vianpyro]

Before: 😰

LEAK SUMMARY:
   definitely lost: 9,373,704 bytes in 250,699 blocks
   indirectly lost: 6,932,840 bytes in 206,429 blocks
     possibly lost: 40 bytes in 1 blocks
   still reachable: 0 bytes in 0 blocks
        suppressed: 0 bytes in 0 blocks

Now: 🥴

LEAK SUMMARY:
   definitely lost: 981,384 bytes in 40,891 blocks
   indirectly lost: 6,931,512 bytes in 206,387 blocks
     possibly lost: 1,368 bytes in 43 blocks
   still reachable: 0 bytes in 0 blocks
        suppressed: 0 bytes in 0 blocks

Command to run Valgrind:

gcc ./*.c ./solve/*.c -o 8-puzzle && valgrind --leak-check=full --show-leak-kinds=all --track-origins=yes --verbose --log-file=valgrind-out.txt ./8-puzzle -p 386257041

valgrind-out.txt