fix: add missing validations and prevent buffer overflow in seccomp loader

[KenzieEdernez]

Changes:

1. Fixed Heap Buffer Overflow: The loader now allocates exactly the memory needed (len_bytes) instead of a fixed 32KB buffer, and verifies that the read size matches the allocation.
2. Added File Validation: Implemented checks for "Magic Bytes" ('S', 'C') and Version number to reject invalid files.
3. Added Size & Alignment Checks:
   • Verified that len_filter does not exceed MAX_BPF_SIZE.
   • Verified that len_filter is a valid multiple of sizeof(struct sock_filter) to ensure instruction alignment.
4. Resource Cleanup* Updated main.c to properly close the file handle and free allocated memory before exiting.