Fix a stack overflow in ifconfig(8).

Fix a stack overflow in ifconfig(8).

The stack overflow happens, if "-" is passed as the argument to 'ssid' or
'wepkey' commands.  The offender is ifieee80211.c:get_string()'s "-" special
handling:
...
	len = p - buf;
	/* The string "-" is treated as the empty string. */
	if (!hexstr && len == 1 && buf[0] == '-')
		len = 0;
	if (len < *lenp)
		memset(p, 0, *lenp - len);
...

If the string is "-", the 'p' will be 1 byte beyound 'buf' and 'len' is set to
0.  'len' must be less than '*lenp' here, so memset() will be called.  But the
length, used to clear the buffer, is 1 byte larger the buffer pointed by 'p'