board/renesas/ecovec/ecovec.c: fix buffer overflow

Fix error detected by cppcheck:

[board/renesas/ecovec/ecovec.c:66]: (error) Buffer is accessed out of
bounds.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Nobuhiro Iwamatsu <iwamatsu.nobuhiro@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>

board/renesas/ecovec/ecovec.c

@@ -41,7 +41,7 @@ static void debug_led(u8 led)
 int board_late_init(void)
 {
 	u8 mac[6];
-	char env_mac[17];
+	char env_mac[18];
 
 	udelay(1000);