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