bgpd: Fix crash with ecommunity string

bgpd: Fix crash with ecommunity string

When we are displaying a extended community ECOMMUNITY_SITE_ORIGIN
the display sprintf is this:

len = sprintf(
	str_buf + str_pnt,
	"EVPN:%02x:%02x:%02x:%02x:%02x:%02x",
	macaddr[0], macaddr[1], macaddr[2],
	macaddr[3], macaddr[4], macaddr[5]);

The problem with this is that macaddr[0] is passed in as a integer
so the sprintf function thinks that the value to display is much
larger than it actually is.  The ECOMMUNITY_STR_DEFAULT_LEN is 27
So the resulting string no-longer fits in memory and we write
off the end of the buffer and can crash.  If we force the
passed in value to be a uint8_t then we get the expected output
since a single byte is displayed as 2 hex characters and the
resulting string fits in str_buf.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>