Fix strncat size argument in send_handshake_query

Found by clang:
client.c:1276:26: warning: the value of the size argument in 'strncat' is too large, might lead to a buffer overflow [-Wstrncat-size]
        strncat(buf, topdomain, sizeof(buf) - strlen(buf));
                                ^~~~~~~~~~~~~~~~~~~~~~~~~

src/client.c

@@ -1273,7 +1273,7 @@ send_handshake_query(int fd, char *prefix)
 	buf[0] = 0;
 	strncat(buf, prefix, 60); /* 63 - space for 3 CMC bytes */
 	strcat(buf, cmc_dot);
-	strncat(buf, topdomain, sizeof(buf) - strlen(buf));
+	strncat(buf, topdomain, sizeof(buf) - strlen(buf) - 1);
 	send_query(fd, buf);
 }