In aesXCBCmac fix (potential) out of bound write in padding.
CID 155904 (#1 of 1): Out-of-bounds write (OVERRUN)
7. overrun-local: Overrunning array of 16 bytes at byte offset 16 by dereferencing pointer p2++.
CID 155910 (#1 of 1): Out-of-bounds access (OVERRUN)1.
overrun-buffer-val: Overrunning buffer pointed to by key of 7 bytes by passing it to a function which accesses it at byte offset 63.