[Bountybette]

With the OpenSSL backend, usbguard::Hash holds the digest context as an
owning EVP_MD_CTX*, but the copy constructor copied it by value:

_state = rhs._state;

so a copy and its source share one heap context. The hashing path copies a
Hash (Hash hash(_hash); in DevicePrivate::updateHash); when that copy is
destroyed while the source is still live, EVP_MD_CTX_free releases the
context the source still points at.

ASan on the copy-then-use path:

==ERROR: AddressSanitizer: heap-use-after-free ... in EVP_DigestFinal_ex
  #2 usbguard::Hash::getBase64() Hash.cpp:210
freed by thread T0 here:
  #1 EVP_MD_CTX_free
  #2 usbguard::Hash::~Hash() Hash.cpp:119

The libgcrypt backend already deep-copies with gcry_md_copy and libsodium
keeps the state by value; only OpenSSL aliased. The copy constructor now
allocates a fresh context and copies into it with EVP_MD_CTX_copy_ex,
matching the default constructor. Added test_Hash.cpp asserting a copy is
independent of its source (fails under the asan job before this change).