Fix possible NULL pointer dereference in sha2_mac_init()

If mechanism->cm_param is NULL, passing mechanism to
PROV_SHA2_GET_DIGEST_LEN() will dereference a NULL pointer.

Coverity reported this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14044