tools: kwboot: Initialize rfds to zero

Explicitly zero out the rfds fd_set with FD_ZERO() before using it.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>

tools/kwboot.c

@@ -1151,6 +1151,7 @@ kwboot_terminal(int tty)
 		fd_set rfds;
 		int nfds = 0;
 
+		FD_ZERO(&rfds);
 		FD_SET(tty, &rfds);
 		nfds = nfds < tty ? tty : nfds;