mgmt/osdp: Make sure we decrypt atleast one block

OSDP secure channel message blocks are terminated with a 0x80. This means
that even for zero length messages, we would have at least one block of
encrypted data to decrypt (since message blocks are rounded up to the next
16 byte boundary). The current length assertion checks for 16 byte
alignment but a malicious peer could send a specially crafted packet with
zero length blocks. Fix this issue by adding check for length == 0 case.

Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>