iscsi-scst: Fix wrong variable in chap_calc_digest_af_alg memcpy

Use 'bytes' (the return value from af_alg_final) instead of 'res'
(which is 0 after the last successful af_alg_update call) when
copying the digest.

This bug caused the memcpy to copy 0 bytes, resulting in an
uninitialized digest buffer. It also triggered a GCC
-Werror=stringop-overflow warning because 'res' could theoretically
be negative, leading to a huge unsigned size.

iscsi-scst/usr/chap.c

@@ -380,7 +380,7 @@ chap_calc_digest_af_alg(char *alg, char chap_id,
 		goto out;
 	}
 
-	memcpy(digest, buffer, MIN(res, digest_len));
+	memcpy(digest, buffer, MIN(bytes, digest_len));
 
 out:
 	close(datafd);