smb: client: fix next_buffer UAF and NextCommand bounds in compound PDUs

Fix several related bounds checking and pointer lifecycle issues in
receive_encrypted_standard()'s handling of compound encrypted frames:

- Clear next_buffer after assigning it to server->bigbuf. A stale
  next_buffer pointer can lead to a use-after-free on subsequent
  error paths.
- Update pdu_length to the decrypted plaintext size (buf_size). Using
  the pre-decryption length allows NextCommand to point into stale
  ciphertext residue.
- Reject next_cmd values smaller than MID_HEADER_SIZE(server).
- Fix an integer overflow in the upper bound check by verifying
  pdu_length - next_cmd < MID_HEADER_SIZE(server), ensuring the
  trailing slice is large enough for a header.

Fixes: b24df3e30cbf ("cifs: update receive_encrypted_standard to handle compounded responses")
Cc: stable@vger.kernel.org
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>