[rng] Fix build error when assertions are enabled

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/drbg.c

 		       state, strerror ( rc ) );
 		return rc;
 	}
-	assert ( len >= min_len );
-	assert ( len <= sizeof ( data ) );
+	assert ( len >= ( int ) min_len );
+	assert ( len <= ( int ) sizeof ( data ) );
 
 	/* 9.  initial_working_state = Instantiate_algorithm (
 	 *     entropy_input, nonce, personalization_string ).