pefile: Fix the failure of calculation for digest

Commit e68503bd forgot to set digest_len and thus cause the following
error reported by kexec when launching a crash kernel:

	kexec_file_load failed: Bad message

Fixes: e68503bd (KEYS: Generalise system_verify_data() to provide access to internal content)
Signed-off-by: Lans Zhang <jia.zhang@windriver.com>
Tested-by: Dave Young <dyoung@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
cc: kexec@lists.infradead.org
cc: linux-crypto@vger.kernel.org
Signed-off-by: James Morris <james.l.morris@oracle.com>