lua net.interfaces(): fix mac addresses

Only the first byte was being shown since 3ab77332.
I can't see at all why this part was changed;
the buffer is (and was) way overlong for this,
so writing one zero byte just after the end is OK.

daemon/bindings/net.c

@@ -240,11 +240,11 @@ static int net_interfaces(lua_State *L)
 
 		/* Hardware address. */
 		char *p = buf;
-		memset(buf, 0, sizeof(buf));
-		for (unsigned k = 0; k < sizeof(iface.phys_addr); ++k) {
-			sprintf(p, "%s%.2x", k > 0 ? ":" : "", iface.phys_addr[k] & 0xff);
+		for (int k = 0; k < sizeof(iface.phys_addr); ++k) {
+			sprintf(p, "%.2x:", (uint8_t)iface.phys_addr[k]);
 			p += 3;
 		}
+		p[-1] = '\0';
 		lua_pushstring(L, buf);
 		lua_setfield(L, -2, "mac");