fix tricky signed/unsigned comparison (ITS#2835)

libraries/liblutil/passwd.c

@@ -485,7 +485,7 @@ static int chk_ssha1(
 
 	rc = lutil_b64_pton(passwd->bv_val, orig_pass, passwd->bv_len);
 
-	if (rc <= sizeof(SHA1digest)) {
+	if (rc < 0 || (unsigned)rc <= sizeof(SHA1digest)) {
 		ber_memfree(orig_pass);
 		return -1;
 	}
@@ -566,7 +566,7 @@ static int chk_smd5(
 
 	rc = lutil_b64_pton(passwd->bv_val, orig_pass, passwd->bv_len);
 
-	if (rc <= sizeof(MD5digest)) {
+	if (rc < 0 || (unsigned)rc <= sizeof(MD5digest)) {
 		ber_memfree(orig_pass);
 		return -1;
 	}